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Many Democrats hope that Tim Kaine, chairman of the Democratic National Committee and a former Virginia governor, will consider a bid for the seat, given his previous success in statewide races. But Mr. Kaine has expressed little interest in serving in the Senate.

Asked about the possibility of Mr. Kaine rethinking a run, Mr. Moran answered, “At this time, I think everyone has to re-evaluate their positions on this race now that the senator has decided not to run.” Mr. Moran is a former state delegate from Alexandria and a 2009 gubernatorial-primary candidate.

Brian Walsh, a spokesman for the NRSC, said that Democrats “will have a great difficulty finding an electable candidate for this open seat, as Virginians continue to reject their agenda of higher taxes and reckless spending.”

“We can only hope that Democrats succeed in recruiting President Obama’s No. 1 cheerleader in Washington — Tim Kaine,” he said.

But Republicans still have some questions to answer, as Mr. Allen faces opposition from Jamie Radtke, a tea party member who already has begun to challenge Mr. Allen’s record on spending in Congress.

Mo Elleithee, a Democratic strategist, said that anybody who is “writing off this seat for Democrats doesn’t know Virginia” and overlooking the fact that Mr. Allen has to survive a party primary.

“He is going to have to run to the right to placate the tea party crowd, and Virginians typically don’t like far-right candidates for statewide elections,” he said.