Frank Lautenberg, 89, the defiantly liberal New Jersey senator who rose from working-class roots, built a business that made him a multimillionaire and represented his home state for nearly 30 years, died Monday morning in Manhattan.

Mr. Lautenberg, of Cliffside Park, N.J., was the oldest member of the Senate and the only remaining World War II veteran in the chamber.

He died of complications from viral pneumonia at 4:02 a.m. at NewYork-Presbyterian Hospital/Weill Cornell, his office said.

New Jersey Gov. Christie, a Republican, will appoint a replacement for the Democratic senator, likely altering the balance of political power in the closely-divided chamber for the near term.

In Washington, President Obama issued a statement saying: "Michelle and I were deeply saddened to learn of the passing of Senator Frank Lautenberg, a proud New Jerseyan who lived America's promise as a citizen, and fought to keep that promise alive as a senator."

Mr. Lautenberg, proud and combative, was a self-made man who fought for public safety initiatives - waging battles to ban smoking on planes, set a national minimum drinking age, strengthen environmental protection and tighten gun laws - while relishing sharp-edged politics right to the end of his career.

He had already decided not to seek reelection in 2014.

Mr. Lautenberg loved the prestige of the Senate and jousting with powerful political opponents, focusing lately on taking on the gun lobby. On April 17, as the Senate considered a series of bills to strengthen gun laws, an ailing Mr. Lautenberg was wheeled onto the chamber's floor to raise his hand and vote "aye."

That day, Sen. Richard Blumenthal (D., Conn.) called him "a great champion and a warrior." It was one of the last times Mr. Lautenberg cast a vote in Congress.

The son of Russian and Polish immigrants whose father died when he was 18, Mr. Lautenberg lived a life that allies often described as quintessentially American. He wanted to see others have the same opportunities that helped him, friends said.

"He lived the American dream and he valued that," said Brad Lawrence, a political consultant who worked on Mr. Lautenberg's 2002 campaign, and wanted to ensure that others could follow a similar path to success.

"He came from very humble background, he became very wealthy, but he really never forgot his roots," said U.S. Rep. Frank Pallone (D., N.J.), who volunteered on Mr. Lautenberg's first campaign, in 1982, and worked closely with the senator in Washington.

Mr. Lautenberg, a former smoker who quit at the urging of one of his daughters, sponsored landmark bills banning smoking on domestic flights and raising the national drinking age to 21.

He was an advocate for the families of those killed in Pan Am Flight 103 and pushed for heightened airport security and tougher environmental regulations.

His 1987 law barring smoking on short domestic flights opened the door for the no-smoking laws that have become common in public spaces across the country.

He was liberal on the most critical issues of his day. He spoke out against the second Iraq war (though he was not in office when it was authorized), opposed restrictions on abortion and favored expanded rights for same-sex couples.

Friends said Mr. Lautenberg's drive for tighter environmental laws and worker protections were fueled by the death of his father, Samuel, from colon cancer.

After that, he told the New York Times in 2002, "our whole life became so enveloped with sadness."

Always, he worked to bring transportation funding back to his traffic-congested state. A major rail hub in Secaucus bears his name.

"Frank is a steadfast champion of the people of New Jersey," President Obama said in a statement after Mr. Lautenberg announced his impending retirement in February. "His service in World War II is a testament to his character and deep commitment to public service."

Mr. Lautenberg loved the Senate limelight and skiing at ritzy Colorado destinations, but friends also recalled a working-class side that adored New Jersey. Former aide Karen Elkis remembered a campaign ad in his which Mr. Lautenberg bit into a New Jersey tomato. After events at the shore, Pallone said, the senator would insist on a side trip to Max's Famous Hot Dogs in Long Branch.

Despite his long tenure, Mr. Lautenberg never acquired the power or broad influence that comes with a committee chairmanship, in large part because he lost his seniority after his first Senate retirement in 2000. Instead, Mr. Lautenberg was known as a nuts-and-bolts lawmaker, more workmanlike than visionary.

As he completed his fifth Senate term, health problems diminished his role in Washington. He had overcome stomach cancer in 2010, but recently was absent for long stints. A bout of the flu and bronchitis in late 2012 forced him to miss votes on a relief package to help New Jersey recover from superstorm Sandy, and he was out for more than a month again earlier this year as the Senate took up the debate on guns, the issue he had long championed.

He began using a cane, as leg pain and fatigue crept in, and was eventually confined to a wheelchair.

He did not speak on the Senate floor in April when a vote came up on his plan to limit the size of gun magazines. Blumenthal made the case instead.

"He always struck me as someone very proud of his success, both in business and in politics," said David Eichenbaum, a media consultant who ran Mr. Lautenberg's 1994 campaign. "He was a personality that was fierce and fiercely proud."

Mr. Lautenberg's campaigns took on that personality, Eichenbaum said.

"He never wanted any attack to go unchallenged," Eichenbaum said.

His pride led to sharp clashes with allies and opponents alike. He had notoriously frosty relationships with fellow New Jersey senators, including an open feud with Democrat Robert Torricelli, and waged ferocious political campaigns.

One early opponent labeled him a "swamp dog," a label that stuck throughout his career.

As ambitious New Jersey Democrats maneuvered to replace him in 2012, Mr. Lautenberg suggested that the most forward of them, Newark Mayor Cory Booker, deserved the kind of "spanking" he would give his children for disrespect.

U.S. Senator was the only elected office Mr. Lautenberg ever held. He served from 1982 to through 2000, retired and almost immediately regretted the decision. He missed the job, and got a chance to return in 2002, added to the ballot as a replacement after Torricelli's campaign imploded in scandal.

Unlike many elected officials, Mr. Lautenberg didn't come up through the party ranks. Wealth paved his path to politics, but only after growing up with little.

He was born in the working-class city of Paterson, the son immigrants.

"I was born to a poor, working-class family," Mr. Lautenberg said in a 2005 interview with Rutgers University historians. "My father worked in the silk mills of Paterson, as did his brother, as did his father, as much of the city of Paterson residents."

His family, he said, "struggled to make a living."

They lived on such a tight budget that on his 13th birthday his parents argued over a bicycle his mother bought as a gift, Mr. Lautenberg told Rutgers. Tears rolled down his face, he recalled 68 years later, as his father convinced her to return it.

"We couldn't obligate ourselves like that, a dollar a week. So, that stuck in my mind," he told Rutgers.

His early life was unsettled - he lived in some 13 towns as a child, he said in the Rutgers interview. Mr. Lautenberg's father worked a variety of jobs, including selling coal and taking work in silk mills.

"Frank Lautenberg could have been a kid still in Paterson, but (he's) tenacious," Elkis said. "He'd work around the clock to do what he had to do."

Mr. Lautenberg, who graduated from Nutley High School, worked for a time in a luncheonette run by his mother, Mollie, and enlisted, serving in Europe in the Army Signal Corps.

In France, he was part of a team responsible for operating switchboards and other communications infrastructure. After the Army, he attended Columbia University on the GI Bill.

"That was a life changing experience," he said in the Rutgers interview. Later in the talk, he added, "It wasn't the subjects; it was the horizon that it gave me, and that's what has to be remembered. If you give nutrition to a plant, it grows. If you give nutrition to a child, it grows. When you take a mind that can learn and you give it nutrition, by giving it an opportunity to learn, it grows."

"I went from the back of a store, to sitting at the desk that was occupied by Harry Truman before me. I still have the same desk," he said.

Mr. Lautenberg graduated from Columbia's business school in 1949. Dwight D. Eisenhower, then the school's president, handed him his diploma.

A few years later, after selling insurance, Mr. Lautenberg joined two friends who had begun a company processing payrolls. "I thought it was a brilliant idea," Mr. Lautenberg told Rutgers.

He helped build the fledgling company - Automatic Data Processing, or ADP - into a giant. The company went public in 1961 and made Mr. Lautenberg rich: his net worth as of 2010 was valued at more than $40 million, according to the 2010 Almanac of American Politics.

Mr. Lautenberg rarely missed a chance to mention his military service and ADP. He was generous with his fortune, giving heavily to a cancer center in Israel that bears his name and to Columbia, among other causes, and once chaired the philanthropic United Jewish Appeal.

He had a hand in politics long enough to have been on President Nixon's "enemies list;" he had donated $90,000 to George McGovern, the Democratic presidential candidate in 1972.

But he didn't jump into the fray himself until 1982, when, at 58, he decided to run for an open Senate seat.

He'd never been elected to office. He was serving his fourth year on the commission of the Port Authority of New York and New Jersey.

But that year the Abscam bribery scandal churned the New Jersey political world. Democratic Sen. Harrison William resigned before the chamber could expel him.

Mr. Lautenberg poured more than $5 million into the race, narrowly defeating two-term Republican U.S. Rep. Millicent Fenwick, a colorful and independent member of the House who refused donations from corporations or political action committees. Although Mr. Lautenberg chafed at the accusation that he made a campaign issue of Fenwick's age - she was then 72 - political historians say he painted her as an eccentric out of touch with New Jerseyans and questioned her "fitness" to do the job.

"It was really a tough kind of thing, that I have to run against such an elegant person, and do what I did, which was to suggest that coming in as a freshman senator, she at age 72, would have a long way to go before she could be effective," he told Rutgers in 2005.

In 1988, seeking his second term, Mr. Lautenberg faced a challenge from Pete Dawkins, a Vietnam veteran, retired general, multimillionaire and one-time Heisman Trophy winner. The Chicago Tribune called the race "a mudslide."

Mr. Lautenberg had help from Democratic consultants Paul Begala and James Carville, who would go on to national renown four years later when they would guide Bill Clinton's successful campaign for the presidency.

It was Dawkins who dubbed Mr. Lautenberg a "swamp dog."

Allies had a different name. They called him a "liberal lion."

In 2002, Mr. Lautenberg decided to leave the Senate. Friends and relatives had urged him to go out on top, Elkis, the former aide, said.

"He was never happy after he left the Senate, it was the worst decision he ever made," she said. "He missed the ability to help people. He was always a doer."

Mr. Lautenberg also missed the status and excitement of life as a senator, but he would soon have a chance to return, thanks to missteps by his old rival, Torricelli, who was dumped by New Jersey Democrats in 2002 as his poll numbers tanked amid ethics charges.

Well-known and able to self finance a campaign, Mr. Lautenberg, then-78, spent $1.5 million of his own money to beat Republican Doug Forrester.

He was elected shortly after the Senate had voted to authorize the use of force in Iraq. Mr. Lautenberg strongly opposed the war and displayed photos of troops killed there in a foyer outside his Senate office.

He also took immense pride in his long tenure.

"There's a courthouse here that carries my name. There's a railroad station that carries my name, and I'm proud of the service I gave," he told Rutgers.

On the day he announced that his current term would be his last, Mr. Lautenberg left the Senate floor by a side-door and said he felt "disconsolate."

"I liked serving here," he said.

Mr. Lautenberg is survived by his wife, Bonnie Englebardt Lautenberg; six children and their spouses, Ellen Lautenberg and Doug Hendel; Nan and Joe Morgart; Josh and Christina Lautenberg; Lisa and Doug Birer; Danielle Englebardt and Stuart Katzoff; and Lara Englebardt Metz and Corey Metz and 13 grandchildren.