Democratic Senator Robert Torricelli has dropped his bid for re-election, complicating his party's attempts to hold onto its Senate majority in November's mid-term elections.

Torricelli: Apologised to voters but denied any wrongdoing

Mr Torricelli, who has been dogged by questions about his ethics, made his announcement in an emotional resignation speech on Monday.

Party officials say they will announce a new candidate for his New Jersey seat within 48 hours. But the Republicans are set to mount a legal challenge to block his replacement on the ballot papers after the deadline.

The New Jersey state superior court is due to hold a hearing later on Tuesday as the Democrats, who have currently one-seat majority in the Senate, petition to have a new name added to the ballot.

Mr Torricelli made the decision to step down after he was reprimanded by the Senate Ethics Committee in July this year for accepting improper gifts from businessman David Chang and then lobbying on his behalf.

"It is the most painful thing I've ever done in my life," he said at a news conference, at times seeming close to tears.

"I apologise to those who've fought so hard and believed in me and all the causes that I value," Mr Torricelli said, adding that he had done no wrong.

'Desperate attempt'

No charges were upheld against the senator, due to Mr Chang's perceived unreliability as a witness.

However last week a leaked memo from federal investigators said they had found "substantial corroborating evidence" for Mr Chang's claims, French agency AFP reported.

It was the final blow for Mr Torricelli's campaign.

His Republican opponent in New Jersey, Doug Forrester, has vowed to challenge any attempt to replace names on the ballot papers, calling the action a "desperate attempt to retain power".

"The laws of the state of New Jersey do not include a 'we think we're going to lose so we get to pick someone new' clause," he said.

The Democrats currently hold 50 Senate seats, with Republicans controlling 49 Senate seats and Republican Senator James Jeffords of Vermont declared as an independent.