WASHINGTON—Democrats for the first time are acknowledging that Republicans could retake the Senate this November if everything falls into place for the GOP, less than two years after Democrats held a daunting 60-seat majority.

Leaders of both parties have believed for months that Republicans could win the House, where every lawmaker faces re-election. But a change of party control in the Senate, where only a third of the members are running and Republicans must capture 10 seats, seemed out of the question.

That's no longer the case. The emergence of competitive Republican candidates in Wisconsin, Washington and California—Democratic-leaning states where polls now show tight races—bring the number of seats that Republicans could seize from the Democrats to 11.

Democrats now control the Senate 59-41—after the death of Democratic Sen. Edward M. Kennedy, who was replaced by Republican Sen. Scott Brown—including two independents who usually vote with them. That means Republicans need 10 seats to take a 51-49 advantage.

Republicans would have to win virtually every competitive race to retake the Senate, without losing any seats of their own—clearly an uphill climb. The trouble for Democrats is that many trends are against them. Surveys show that Republicans are more motivated than Democrats to go to the polls, and that voters are looking for new leadership in Congress.

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"I think there is definitely a chance" of losing the Senate, said Democratic strategist Gary Nordlinger, a Washington-based media consultant. "I wouldn't call it a probability, but there is certainly a chance."

"Republicans still have to [win] all the competitive races in order to get to a majority, but at least there are enough seats on the table to pull it off," said Nathan Gonzales, political editor of the non-partisan Rothenberg Political Report.

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