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In Florida’s heavily Republican 16th District, for example, where Democrat Tim Mahoney narrowly won the seat after Republican Rep. Mark Foley resigned in the House page sex sandal, the Republican Party is now widely favored to get the seat back.

In Georgia’s 8th District, Democrat Jim Marshall squeaked through with less than 1,800 votes in what Mr. Spain called “the worst Republican environment since Watergate.” The Republican Party is rallying behind Rick Goddard, the former commanding general at the Warner Robins Air Logistics base, who is expected to give Mr. Marshall a tough race.

In Texas’s 22nd District, which gave Mr. Bush 64 percent of its vote in 2004, Democrat Nick Lampson won by a slim margin, even though his Republican opponent was not on the ballot. “This is probably the GOP’s best opportunity to take back a district they lost last year,” Mr. Rothenberg said.

“In order to win back the majority, we don’t have to conquer new territory. We just need to reclaim old territory,” Mr. Spain said.