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His approval rating remains astronomical among black voters, but the question is whether Mr. Obama and congressional Democrats can translate that support into votes.

Former Rep. Thomas M. Davis III, who ran the House Republicans’ campaign committee in 2000 and 2002, said Republicans are aware of the danger of awakening the massive base of black voters. He said that’s one reason so many Republican attacks are against House Speaker Nancy Pelosi rather than against Mr. Obama.

“Hitting Obama can stir up part of their base. The African-American base right now is napping. They’re asleep,” he said. “Democrats are just waiting for somebody out there to say something — they used to call it waving the bloody shirt, is what they called it — to get the old animosities and antagonism going.”

One Democratic strategist said that the black vote will “make the difference” in gubernatorial and House races in Florida, Georgia and Ohio, and could be a factor elsewhere.

“It could decide the Texas governor’s race,” the strategist said. “The Texas governor’s race a lot of people think is actually within reach for Democrats if black turnout in Houston is up.”