Advisers to Mr. McCain said they thought the problems Congressional Republicans were having would not translate into significant problems for Mr. McCain. But they said it steeled their resolve to run a campaign that distinguished Mr. McCain from both Mr. Bush and a Congress where he has served, in the House and the Senate, since January 1983. They said Mr. McCain would seek  sometimes explicitly, sometimes not  to distance himself by speaking critically of what he has described as excessive spending in Washington, as well on issues like the environment.

“There’s no question that the results in these special elections portend ominously for House Republicans, but they will have little impact on the presidential election campaign,” said Steve Schmidt, a senior adviser to Mr. McCain.

The special election results left Democrats and Republicans in rare agreement about one thing: President Bush looms as a drag on Republicans. Democratic leaders said a combination of anxiety among voters about the state of the country and the prospect of an unusually heavy turnout of African-Americans meant that many new Senate and House seats could be in play, including those in states like North Carolina that just two years ago seemed out of reach for Democrats.

Woody Jenkins, a Louisiana Republican who lost in a special House election this month, said in an interview that the high African-American turnout in his district was “probably the decisive factor” in his loss.

The election results also raised questions about what had been a main Republican strategy for the fall, if Mr. Obama wins the nomination: to link Democrats in conservative districts to Mr. Obama. Mr. Obama, campaigning in Sterling Heights, Mich., said the outcome in the Mississippi contest, to fill a “hard-core Republican seat,” proved that the strategy would not work.

“They lost it by eight points, and they did everything they could,” Mr. Obama said. “They ran ads with my face on it, and they said, ‘Oh, you look at this, a former liberal, and his former pastor’s said offensive things. They were trying to do everything in the book to try to scare folks in Mississippi, and it didn’t work.”

But Mr. Duncan, the Republican national chairman, said he thought the strategy would be effective as voters became aware of Mr. Obama’s liberal record in the months ahead.