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.”

The defeat is certain to put a damper on plans by Congressional Republican plans to roll out their new policy agenda this week in an effort to turn around their fortunes.

Several House Republicans, who were already scheduled to meet on Wednesday, have said privately they do not see a wholesale leadership shake-up or an overhaul of their campaign operation as a strong option, given that the election is just six months away. But they are likely to consider some changes in response to the Mississippi defeat. Former Speaker Newt Gingrich warned last week that Republicans need a major shift to forestall heavy losses in November.

When Mr. Childers is sworn in, the House will have 236 Democrats to 199 Republicans. The seat was vacated when Representative Roger Wicker, a Republican, was appointed to succeed former Senator Trent Lott.

In the end, tying the white Democrat to the black presidential candidate may have helped Mr. Childers more than it hurt him, as campaign aides reported heavy black turnout, heavier than in a vote three weeks ago when he came within 400 votes of winning.

“I like what Childers was saying: he was more truthful and down to earth,” said Mary Shelton, an African-American who had just voted for him at the Yalobusha County courthouse here.

And Mr. Childers’s association with the party that might nominate Mr. Obama didn’t hurt either. “We need a change, we really do,” Ms. Shelton said.