Leahy, a superdelegate and supporter of Barack Obama, acted the same day that Democratic Chairman Howard Dean urged superdelegates to announce their presidential choices by July 1, The Los Angeles Times reported.
"There is no way that Senator Clinton is going to win enough delegates to get the nomination," Leahy told Vermont Public Radio in his home state. "She ought to withdraw, and she ought to be backing Senator Obama."
Dean, in an interview on "Early Show" on CBS, said he had told both candidates that they should avoid personal attacks that might hurt the party in November. He said he wanted the superdelegates to name their choices.
"There's 800 of them, and 450 have already said who they're for," Dean said. "I'd like the other 350 to say who they're for at some point between now and the first of July so we don't have to take this into the convention."