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With less than nine months to go before Election Day, Republicans are solidly ahead to take at least five seats now held by Democrats - in North Dakota, Delaware, Nevada, Arkansas and Pennsylvania. Five others - Colorado, Illinois, Wisconsin, Indiana and New York - are now fully in play.

Mr. Bayh became the fifth Democratic senator to announce that he will not seek re-election. He joins Sens. Christopher J. Dodd of Connecticut, Byron L. Dorgan of North Dakota, Ted Kaufman of Delaware and Roland W. Burris of Illinois. In a surprising turn of events, the Illinois seat once held by Mr. Obama is now in play, with Republican Rep. Mark Steven Kirk leading Democratic State Treasurer Alexi Giannoulias by six percentage points, according to the latest Rasmussen survey.

The retirements have been announced suddenly as polls show that voters increasingly oppose Mr. Obama’s massive health care reform plan and the prospect of huge federal budget deficits far into the future. Surveys taken in the run-up to last month’s Massachusetts special election - in which Republican Scott Brown won the seat held by Sen. Edward M. Kennedy - found strong voter anger over gridlock in Washington.

Mr. Obama is aware of voter dissatisfaction, and two weeks ago took the extraordinary step of allowing embattled Democrats to sternly question him about his policies. During a televised policy conference of Senate Democrats, he took questions almost exclusively from those locked in tough re-election campaigns - Mr. Reid, Mr. Bennet, Pennsylvania’s Arlen Specter, Sen. Blanche Lincoln of Arkansas, New York’s Kirsten Gillibrand, California’s Barbara Boxer and Mr. Bayh.

Mr. Bayh and Mrs. Lincoln pointedly questioned Mr. Obama over his willingness to seek bipartisan solutions and work with moderates and Republicans. Mrs. Lincoln, trailing badly in her bid for re-election, urged Mr. Obama to “push back” against ideological elements within the Democratic Party.

Her campaign Web site was quick to highlight the exchange to Arkansas voters, distributing a news report on the event headlined “Lincoln challenges Obama over liberal ‘extremes.’ ”