“Very often the instinct for an incumbent party is to defend and justify,” said Geoff Garin, a pollster for Democratic candidates. “But in this kind of environment, the best defense is a good offense. This needs to be a cycle where Republican vulnerabilities are a central part of the debate.”

Charlie Cook, the nonpartisan political handicapper, framed the Democrats’ challenge for 2010 more bluntly. It does not track the genial, new-politics identity that President Obama has cultivated.

“They’re going to have to play really rough,” said Mr. Cook, who pegs Democrats’ chances for holding the House next year at only slightly better than even. “For the average Democratic Congressional incumbent, the opposition researcher will be the most important person in the campaign.”