U.S. Sen. Frank Lautenberg's announcement last week that he has non-Hodgkin's lymphoma returned the spotlight to the issue of succession if he cannot finish his term and the potential impact on partisan control of the Senate.

Under New Jersey law, Republican Gov. Christie can name a replacement for the Democrat to serve until the next general election. If Christie follows the last two governors who had that opportunity, he will pick someone from his party.

In the fall election, 37 Senate seats are open, and analysts expect Republicans to make significant gains. The Senate stands at 58 Democrats, one independent, and 41 Republicans.

Both parties have vulnerable seats, but in the end, the GOP could pick up eight, according to analysts.

"Everyone hopes he recovers fully, but if for some reason he has to retire, it's an automatic pickup for Republicans and could easily be the ninth seat," said University of Virginia political scientist Larry Sabato, an expert on electoral politics.

In 2005, Gov.-elect Jon Corzine picked fellow Democrat Robert Menendez to replace him in the Senate. In 1982, Republican Gov. Thomas H. Kean appointed Nick Brady to replace Democratic Sen. Harrison Williams, who resigned in a corruption scandal.

Worried that the 86-year-old Lautenberg might not complete his term, which expires in 2015, Democratic state legislators tried late last year to pass bills that would replace a departing U.S. senator in a special election or force the governor to appoint a replacement from the senator's party. The bills died before getting a hearing.

But talk of succession took on a more earnest tone last week after Lautenberg's office announced he has a form of stomach cancer.

Doctors at Mount Sinai Medical Center in New York diagnosed B-cell lymphoma. Lautenberg's staff would not detail the extent of his illness but said he had started chemotherapy.

B-cell lymphoma is a type of non-Hodgkin's lymphoma that nearly 66,000 people in the United States had developed by last year, the American Cancer Society estimated. It estimated that 19,500 people died of non-Hodgkin's lymphomas in 2009.

One of the more common forms of this cancer is large B-cell lymphoma, which accounts for one-third of non-Hodgkin's cases and often responds to chemotherapy. About half of those treated are cured.

On Monday, Lautenberg fell in his Cliffside Park, N.J., condominium after a busy week, which included a trip to Haiti to survey earthquake-relief efforts.

His doctors and staff said they expected him to be working in the Senate in between cancer treatments.

Democrats have become increasingly concerned about their grip on the Senate. On Monday, Sen. Evan Bayh (D., Ind.) said he would not seek reelection despite a strong lead in the polls. Last month, Republican Scott Brown overtook Democrat Martha Coakley in the Senate race in Massachusetts, which had been considered an easy Democratic victory.

Other trouble spots for Democrats include Delaware, with the announcement that Attorney General Beau Biden will not seek to follow his father, Vice President Biden, into the Senate; and Pennsylvania, where Sen. Arlen Specter, who became a Democrat last year, faces a tough challenge.

Democrats also have problems in Colorado and Illinois, but Republicans have tough fights in Kentucky, Ohio, and Missouri.

If a Lautenberg retirement raised the chances of a Republican Senate takeover, "I can guarantee the Republicans would open up many fronts" to get control, Sabato said.

Likely hot spots would be California, Wisconsin, and Washington in a race to add that 10th seat.

But Sabato and others caution that an awful lot of ifs are built into the scenario, including the economy and whether Republicans field strong candidates and have the money to fund them.

New Jersey could face the prospect of a Republican governor and a junior Republican senator, which an emboldened GOP could parlay into enhanced fund-raising, which in turn could help its candidates win local races.

A replacement would create an even stickier problem when redistricting is factored in, Rider University political scientist Ben Dworkin said.

Christie's office declined to comment on succession scenarios. "If New Jersey loses a seat in the census and goes from 13 to 12" members of the House because of declining population, Dworkin said, "someone's going to have to go."

If Lautenberg resigns, "suddenly it's easy, when you get to kick somebody upstairs."

But when discussing the future of New Jersey's congressional delegation, Dworkin said, the central factor - Lautenberg - can't be counted out.

"There's no reason to suspect that this is going to really sideline Frank Lautenberg in any way," Dworkin said. "This man won reelection at his age and still skis, which people like me in their 40s can't do."

Born poor in Paterson, N.J., on Jan. 23, 1924, Lautenberg is a self-made multimillionaire.

A quick-witted, staunch liberal, he is a fierce advocate for transportation projects and gun control and a harsh critic of the tobacco industry.

A World War II veteran of the Army Signal Corps, Lautenberg attended Columbia University on the GI Bill. With two friends in Paterson, he started Automatic Data Processing, a worldwide payroll-processing firm that went public in 1961.

Though he worked his way up from the bottom in business, he started his political life near the top: His Senate seat, which he first won in 1982, was his first electoral office. He retired in 2001 but immediately regretted it, missing the Senate and Washington lifestyle, and he leaped at the chance in 2002 to replace the scandal-plagued Sen. Robert Torricelli (D., N.J.).

Though some New Jersey Democrats believed he'd stay for only one term, he ran for reelection in 2008.

For the first time, though, his age became a dominant issue in a surprise Democratic primary against Rep. Rob Andrews (D., Camden) and in the general election against former Republican Rep. Dick Zimmer.

Contact staff writer Cynthia Burton at 856-779-3858 or cburton@phillynews.com.

Inquirer staff writer Josh Goldstein contributed to this article.