Newark, N.J., Mayor Cory Booker is set to make official what the political world has long expected: He’ll run for the U.S. Senate seat vacated by Frank Lautenberg, who died Monday.

A source close to the Democratic mayor told Yahoo News Booker was expected to announce Saturday that he’ll enter the special election primary set for Aug. 13. But the source cautioned that the event could be postponed because of heavy rains and flooding from remnants of Tropical Storm Andrea that were forecast for the region over the next 24 hours. The campaign was expected to make a final call on the event later Friday.

Booker announced in December he was exploring a run for Lautenberg’s seat in 2014, and had long been considered the front-runner in that race. But the special election could complicate his plans—in part because several well-funded lawmakers have announced or expressed their interest in running in what looks to be an increasingly crowded Democratic primary. Booker would have to make up financial ground quickly.

On Thursday, Rep. Rush Holt announced he would seek the seat—describing himself as the "best candidate to continue the passionate advocacy for progressive values that Sen. Lautenberg exemplified.”

Several other Democrats are also said to be eying the race, including Rep. Frank Pallone and state Assembly Speaker Sheila Oliver. Beth Mason, a wealthy Hoboken City Council member, is considering financing her own primary bid, according to the Star-Ledger.

There has been less interest on the Republican side. Several high-profile GOP lawmakers have already bypassed the race. State Sens. Tom Kean Jr. and Joseph Kyrillos, who previously waged U.S. Senate races, said they weren’t interested. State Assemblyman Jon Brammick, who had been exploring a bid, said Thursday he wouldn’t run.

That leaves Steve Lonegan, a former mayor of Bogota, N.J., as the only declared candidate in the GOP primary—though Joe Plumeri, a Trenton businessman and minor-league baseball team owner, is said to be considering a bid.

Candidates on both sides have until Monday to qualify for the primary ballot by submitting 1,000 signatures of support. Winners of the primary will advance to the special election set for Oct. 16.