NEWARK, N.J. - Newark Mayor Cory Booker, who formally declared his candidacy for U.S. Senate on Saturday, now finds himself competing in a primary against like-minded Democratic congressmen that will be decided in midsummer, when exceptionally low voter turnout threatens his early advantage.

Booker made his candidacy official at a news conference in New Jersey's largest city, which he has led since 2006. He is vying to fill the seat of U.S. Sen. Frank Lautenberg, who died Monday at age 89.

Reps. Frank Pallone and Rush Holt are also planning to enter the Democratic primary. Candidates have until Monday afternoon to file petitions with the secretary of state.

Booker, 44, said he would travel the state to earn every vote. He'll have to hurry; Gov. Chris Christie set the primary for Aug. 13 and a special election for the balance of Lautenberg's term for Oct. 16.

"We need someone in the United States Senate who's actually had to work on difficult problems, who's actually had to find people jobs, who's actually had people standing in front of their homes and had to work on everything from getting people into food stamp programs to helping young people better afford college," Booker said Saturday. A second kickoff event was planned later in the day.

Pallone and Holt have yet to announce formal plans. Experts say the race could be an interesting one.

"It will be a question of who can get organizational support from county parties or labor - support from those who will knock on doors and get people out to vote," said political analyst Patrick Murray of Monmouth University. Booker has the national profile and Pallone has more money banked - $3.7 million to Booker's $1.9 million, as of the end of March.

The only Republican running so far is Steve Lonegan, a former Bogota mayor who runs the New Jersey office of Americans for Prosperity.

Booker was joined at the news conference by former U.S. Sen. Bill Bradley, a former pro basketball player who for 18 years held the Senate seat Booker is seeking.

Bradley called Booker "the right person for the right office at the right time."

Booker started fundraising for a 2014 Senate campaign after announcing he would not run against Christie for governor, citing his desire to finish his term in Newark, which expires in June 2014.

Booker is considered the early front-runner, though election observers agree anything can happen in a hastily called summer election.

Pallone, 61, has deep union ties. Holt, 64, a former research physicist, has $800,000 on hand.

The winner of the October special election will hold the seat until November 2014, when voters will elect a senator for the regular six-year term.