Mr. Childers, a local courthouse official in Booneville, pulled together a coalition of blacks, who turned out heavily, and old-line “yellow dog” Democrats, to beat his Republican opponent, Greg Davis, the mayor of Southaven, a Memphis suburb. With all precincts reporting, the vote was 54 percent for Mr. Childers to 46 percent for Mr. Davis.

The seat had been in Republican hands since 1995, and the district, largely rural and stretching across the northern top of Mississippi, had been considered one of the safest in the country for President Bush’s party, as he won here with 62 percent of the vote in 2004.

Mimicking a strategy that proved successful in 2006, Democrats ran staunch conservatives in both this and the Louisiana race, forcing their Republican opponents to attack national party figures as surrogates.

Mr. Davis had been hoping for a large turnout in his home of DeSoto County, where roughly 15 percent of the district’s voters live, and which is solidly Republican and mostly white. But a last-minute appearance for him by Mr. Cheney on Monday apparently failed to rally his base sufficiently; indeed a modest room at a local convention center was hardly packed.

“There are indications that the normal Republican turnout is just not there,” Mr. Black said. “If they can’t win up there, where are you going to win?”

Both Mississippi candidates depicted themselves as down-the-line conservatives on social issues, and there was little difference between them on abortion and gun rights: staunchly against the first, and for the second.

But the Republican strategy of trying to link Mr. Childers to more liberal national Democratic figures fell short, as it did in Louisiana. Indeed, voters here were bombarded by advertisements equating Mr. Childers with Senator Barack Obama, a tactic intended to turn conservative whites away from Mr. Childers and which some politicians said played on white racial resentments. Mr. Childers, for his part, fiercely resisted the connection, calling himself over and over a “Mississippi Democrat.”