Republican U.S. Representative Bill Cassidy is challenging Mary Landrieu, a three-term incumbent, who is fighting to remain one of only two Democratic senators from the southeastern U.S. - a party stronghold a generation ago - after losses last month by Democratic incumbents in Arkansas and North Carolina.
Cassidy, who has run a disciplined campaign focused on linking Landrieu to President Barack Obama, who is deeply unpopular among whites in Louisiana, is leading by more than 20 percentage points, according to a RealClearPolitics average of polls in recent weeks.
"It's a campaign that has been very methodical and careful and doesn't do stupid things," said John Couvillon, a Louisiana pollster working mostly for Republicans.
If victorious, Cassidy would be the ninth Republican to capture a previously Democratic seat this year, and would give his party 54 seats in the 100-member Senate. The runoff is being held because no candidate won more than 50 percent of the vote in the Nov. 4 open primary.
Landrieu, the top Democrat on the Senate Energy and Natural Resources Committee, has campaigned on her clout, a message complicated by her party's loss of Senate control and by her recent failed push to win Senate approval of the Keystone XL pipeline from Canada's oil sands to the U.S. Gulf Coast.
This week she seized on allegations that Cassidy, a medical doctor, falsified some time sheets and failed to file others for a part-time Louisiana State University hospital job. Cassidy has denied the allegations, which the school has said it is reviewing.
Landrieu's difficulties have been compounded by the scant financial support she has received from outside groups and Democratic campaign committees ahead of the runoff.
Days before the election, she told the Washington Post that she felt that the Democratic Senatorial Campaign Committee had "walked away from the race."
Cassidy has had no similar problems. Ads in support of him and critical of Landrieu have saturated the airwaves, including one 30-second anti-Landrieu video released by Ending Spending Action Fund in late November featuring nine separate images of Obama.