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Different stories are unfolding in Colorado, where polls show Mr. Bennet trailing Republican Ken Buck, like Ms. O’Donnell a favorite of tea party activists, and in Mr. Obama’s home state of Illinois, where Republican Rep. Mark Steven Kirk is in a dead heat with Democrat Alexi Giannoulias, the Illinois state treasurer.

The Illinois race seemed unlikely two years ago after Mr. Obama resigned his seat to move to the White House. Filling it in such a heavily Democrats state with a Democratic governor seemed to be a mere formality.

But Blagojevich, then about to be ousted in a corruption scandal, sparked a political circus by appointing the obscure Mr. Burris to the seat and getting charged with, among other things, scheming to peddle Mr. Obama’s seat for personal profit, campaign contributions or a lucrative post in the new administration.

The scandal has been a heavy political burden for Democrats in the state, where four of the past eight governors have been convicted of crimes or left office under an ethical cloud.

Kent Redfield, a political science professor at the University of Illinois at Springfield, said that if Mr. Obama still held the Senate seat, “he would essentially just be cruising toward re-election” and that Mr. Kirk likely would not have run.

“The Senate seat would not be in play,” Mr. Redfield said.

The Democratic Senatorial Campaign Committee has responded in the past two weeks by going after Ms. O’Donnell, Mr. Buck and Mr. Kirk, doling out more than $1.7 million in media buys and to cover the costs of online advertising, according to the nonpartisan Center for Responsive Politics.

While the 2008 gains brought problems for the Democrats this year, history suggests the political picture could change, and losses in 2010 could translate into a Democratic win in 2012.

Obama could benefit from political triangulation, as voters may begin to share the blame for economic and government woes between both parties instead of concentrating their frustration on Democrats who currently control the federal government unilaterally,” said Isaac Wood of the University of Virginia Center for Politics.

“If a Republican wave in 2010 begets Obama’s re-election in 2012, that would add yet another layer of irony to a political world already quite familiar to the concept,” he added.