WASHINGTON - Cory Booker is in.

The Newark mayor with a national profile will run for New Jersey's open U.S. Senate seat.

Booker announced his candidacy Saturday in Newark.

His plan to run was confirmed by a campaign aide Friday. His announcement follows a long-expected bid to formally enter national politics.

Booker, a Democrat, planned two Saturday events to make an official announcement, one in Newark and the other Saturday afternoon in Willingboro.

To most political observers, the well-known mayor has been seen for months as the favorite to replace Sen. Frank R. Lautenberg, who had announced he would not seek a new term and who died Monday. Some of Booker's supporters predict that a Senate seat is a step toward even bigger things.

"We think Cory has the leadership skills and values to really move the country forward over the next decade, couple decades," said Steve Phillips, chairman of PAC+, a group that plans to spend more than $1 million mobilizing young and minority voters to help Booker.

"We think there's huge potential. He already has captured the imagination of a diverse, young constituency across the country," Phillips said. "As the country starts to think about and move toward what's going to be the post-Obama progressive leadership, we think he's one of the best-positioned people to move that forward."

First, though, Booker will face a challenge for the Democratic nomination from U.S. Reps. Rush Holt, of central New Jersey, and Frank Pallone, from the Shore.

The primary is scheduled for Aug. 13, with the general election Oct. 16, but on Monday, Peg Schaffer, an attorney and Somerset County Democratic chairwoman, plans to file suit in the Appellate Division on behalf of "three voters and taxpayers" to move the Senate election to the gubernatorial election date in November. She said she filed papers Friday to lay the groundwork for the suit.

The special election would lead to "voter suppression," Schaffer said, because of the confusion sowed by two elections in three weeks. There will be different deadlines for voter registration and mail-in balloting, for example.

The special election is anticipated to cost $11.9 million.

While Pallone has an early lead in campaign funds and each congressman has a long record of electoral success in his districts, neither has the statewide name recognition of Booker, who leads New Jersey's largest city, regularly appears on television, and brought the Democratic faithful to their feet last year with a speech at the party's national convention.

"The reality is that this is Booker's race to lose," said Brigid Harrison, a political science professor at Montclair State University.

Booker would be the ninth black senator in American history and the first from New Jersey. He would join two other black senators currently in the chamber, potentially increasing his appeal, Harrison said.

"He is going to draw considerable resources from outside the district in terms of fund-raising," she said. "He has the potential to mobilize for many voters in New Jersey for whom that's important."

Already, PAC+ has launched a "Help Cory Win" campaign to turn out voters - a key to the race, given the unusual election dates.

After vaulting to prominence through his shining biography, starring roles in a documentary and television series, devoted Twitter following, and penchant for heroics, Booker also boasts well-heeled backing from Hollywood, New York, and Silicon Valley.

Critics, though, question whether his fame matches his results. A potentially bruising campaign could test how well his well-honed image stands up to attacks.

Republicans already are taking aim.

"It remains to be seen whether Cory Booker the candidate can match Cory Booker's celebrity persona in this kind of race," said Brad Dayspring, a spokesman for the GOP's national Senate campaign arm.

At fund-raisers, Booker, 44, has cast himself as a problem-solver who can reach across party lines, while Holt and Pallone have touted their strongly liberal records.

The only well-known Republican in the race is conservative leader Steve Lonegan, who has twice run in primaries for governor. He led the New Jersey chapter of Americans for Prosperity until announcing his candidacy this week.

The Newark Star-Ledger reported Friday that West Orange resident Brian Goldberg and Piscataway doctor Alieta Eck also are collecting signatures for a run. Neither has held public office, the paper reported.

Candidates must file 1,000 signatures by Monday in order to run.

Contact Jonathan Tamari at jtamari@phillynews.com or follow on Twitter @JonathanTamari. Read his blog "Capitol Inq" at www.inquirer.com/CapitolInq.
Matt Katz contributed to this report.