Extending an expensive and largely negative campaign for one more month, voters in Louisiana sent Mary Landrieu, a Democrat and three-term United States senator, and her Republican challenger, United States Representative Bill Cassidy, into a runoff election set for Dec. 6.
Given Louisiana’s nonpartisan primary system — in which all candidates run in a primary and, if no one wins a majority, the top two vote-getters compete in an election a few weeks later — runoffs are fairly routine here and one had long been expected in this race.
While the state Republican Party formally backed Mr. Cassidy this summer, another Republican, Rob Maness, a retired Air Force colonel with a strong Tea Party following, stayed in the race and drew about 14 percent of the vote on Tuesday. Mr, Cassidy received 42 percent of the vote and Ms. Landreau 41 percent.
Numerous polls suggest that much of Mr. Maness’s support will move to Mr. Cassidy in the runoff, putting him in a strong position to win.
Ms. Landrieu is the last Democrat representing a Deep South state in the United States Senate and the last Democrat in office in Louisiana to be elected statewide. She won runoffs in her first and second elections for the Senate, in 1996 and 2002, though neither by huge margins. Given the rightward drift of the state, and particularly of the once reliably Democratic voters of southern Louisiana, Ms. Landrieu may be facing her toughest Senate election to date.
“She’s got to find a way to get to 30 percent” of white voters, said Roy Fletcher, a Republican political consultant in Baton Rouge. “In this headwind, with Obama and et cetera, I’m not sure.”