Republicans lost a long-held House seat Saturday after trying to bring down the Democratic candidate by aligning him with Barack Obama.

Don Cazayoux defeated Woody Jenkins to represent Louisiana's Sixth District, a seat held by Republicans for decades, boosting Democrats' optimism heading in to the fall elections.

Mr. Cazayoux, a state representative, and Mr. Jenkins, a newspaper publisher and well-known former state legislator, were vying for the seat of Republican Rep. Richard Baker, who resigned in February. Mr. Cazayoux's victory will increase House Democrats' majority 235 to 198.

The victory could also aid Sen. Obama in making his case to uncommitted, elected superdelegates wary of how the top of the ticket could affect their own races in November.

Elected superdelegates comprise about half of the 795 officials who cast ballots at the Democrats' national convention to choose the party's presidential nominee.