TRENTON, N.J. ? Dogged by questions about his ethics and falling in the polls, Democratic Sen. Robert Torricelli abruptly dropped his bid for a second term Monday, throwing a twist into the battle for the Senate just five weeks before Election Day.

Democratic officials said they would announce a new candidate within 48 hours. The Republicans said they would file an immediate court challenge to block any attempt to replace Torricelli this close to the election.

The embattled incumbent, who has been in Congress for nearly 20 years, was severely admonished over the summer by the Senate ethics committee after it found he accepted lavish gifts from a campaign contributor. He has denied any wrongdoing even as he apologized to voters.

Torricelli, 51, fought back tears as he announced his decision, noting that he had become an “issue” in the fight for the Senate, where Democrats hold a one-seat majority.

“I could not stand the pain if any failing on my part will do damage to the things and the people that I have fought for all of my life,” Torricelli said, his voice breaking, as Gov. James E. McGreevey and Sen. Jon Corzine stood by his side.

“Don’t feel badly for me,” Torricelli added. “I’ve changed people’s lives. I’m proud of every day of it, and I wouldn’t change a bit of it.”

Republicans said they would contest any effort to have a substitute take Torricelli’s place, saying such a move is barred by state law so close to an election. According to GOP lawyers, the only exception acknowledged by a court has been in the case of the death of a nominee.

Under New Jersey law, a political party can replace a statewide nominee on the ballot if the person drops out at least 48 days before the election. But only 36 days remain until the election, meaning Democrats have to seek approval from the courts.

The party has already asked the state Supreme Court for permission to replace Torricelli’s name on the ballot. State GOP Chairman Joseph Kyrillos said Republicans would file their court challenge today and planned to argue that Torricelli missed the deadline to drop out.

The Republican candidate, Douglas Forrester, said: “The laws of the state of New Jersey do not contain a ‘we think we’re going to lose so we get to pick someone new’ clause.”