A pollster said the outcome had little to do with national political trends, though.
Democratic state Rep. Don Cazayoux polled 49% Saturday to beat GOP publisher Woody Jenkins, who received 46%, for the state's 6th District seat in the U.S. House.
Cazayoux and Jenkins share conservative positions on gun rights and abortion. Interest groups' ads variously accused the Republican of tax evasion and cast the Democrat as a tax-happy liberal.
Some ads painted Cazayoux as a supporter of Democratic presidential contender Barack Obama. Cazayoux responded with a statement saying he had "not endorsed any national politician."
Jenkins spent 28 years in the state House but has failed in attempts at higher office. A polarizing figure, he has displayed plastic fetuses as an antiabortion message.
"Democrats have a hard time winning in Louisiana, except when Woody runs," said Bernie Pinsonat of Baton Rouge-based Southern Media & Opinion Research. "I think Cazayoux got the only Republican opponent he could have beaten."
Cazayoux will be the district's first Democratic congressman in three decades. The seat opened when incumbent Richard H. Baker resigned to become a lobbyist.
Louisiana now has four Republicans and three Democrats in the House.
Its U.S. Senate seats are split.