The move makes Menendez the anointed Democratic candidate in next year's election.
"Congressman Menendez has been a strong leader as the Chairman of the House Democratic Caucus," Sen. Harry Reid, D-Nev. and Senate minority leader, said in a statement. "His leadership skills and experience in public service will make him a great addition to the Senate Democratic Caucus."
The state Republican Party appears to have settled on state Sen. Thomas Kean, R-Morris and son of former Gov. Thomas Kean. Kean will have a hard fight in a state that has not elected a Republican to the Senate in more than three decades.
But a bruising primary fight could make his job easier. Rep. Rob Andrews, who represents the Camden area in South Jersey, may decide to try for the Democratic nomination, and Rep. Frank Pallone, a maverick from the Jersey Shore, could also decide to run.
Menendez brings a $4 million war chest to the race, along with the support of New Jersey's large Hispanic bloc.