With an under-the-klieg-lights U.S. Senate election just days away, incumbent Mary Landrieu and her two leading challengers — U.S. Rep. Bill Cassidy, of Baton Rouge, and retired Air Force Col. Rob Maness, of Madisonville — met in a largely polite debate that broke little new ground in their campaigns.

The candidates wasted no time in reinforcing the overall partisan divide that sets Republicans Cassidy and Maness on one side and Landrieu, a Democrat, on the other: The Republicans bash Barack Obama, the unpopular Democratic president they seek to tie to Landrieu, and she supports many of his policies, if she doesn’t exactly embrace him with fervor.

In response to the first question of the hour-long debate — about the possible use of U.S. ground troops against the Islamic State military force in the Middle East — Cassidy said, “I don’t trust this president. I think he is a terrible commander-in-chief.”

And later, he asked about Landrieu, “My gosh, why does she support the fellow?”

In his response, Maness also said Obama cannot be trusted as commander-in-chief. “Sen. Landrieu has been a rubber stamp for every one of his feckless policies,” Maness said, using an adjective Cassidy would later apply to Obama himself.

Landrieu said all options should be on the table, and that policymakers should listen to military commanders in deciding how to fight the Islamic State .

The debate took place in the Manship School of Mass Communication at LSU. It was sponsored by Raycom Media, which owns dozens of TV stations, including outlets in Baton Rouge, New Orleans, Lake Charles and Shreveport, and was broadcast statewide.

Not surprisingly, the Affordable Care Act also reared its head in the debate, in a discussion about Gov. Bobby Jindal’s rejection of the expansion of Medicaid in Louisiana under the act. The 2010 health insurance reform stands as Obama’s landmark legislation — it’s known as “Obamacare” — and Republicans have sought to capitalize on its unpopularity, as well.

Landrieu called Jindal’s decision a “tragic” denial of health insurance coverage to more than 250,000 Louisiana residents.

Cassidy invoked his experience as a doctor in the charity hospital system and said, “Medicaid is the illusion of coverage without the power of access.

“It is a broken system,” Cassidy said. “We should not expand it.”

Landrieu responded with one of the only pointed comments of the night.

“It’s really stark to me,” she said about Cassidy’s remarks. “He has earned a living from Medicaid” — and now, she said, he was criticizing the program.

Cassidy countered that as a member of the LSU faculty, his paychecks have come from the university.

The debate was the final one of the campaign before the open primary Tuesday. If no one in the eight-candidate U.S. Senate field captures a majority of the Tuesday vote, the top two finishers will meet in a runoff Dec. 6, and one or more additional debates may be scheduled in that event.

Recent polls show Landrieu with a slight edge over Cassidy atop the primary field, but both are short of 50 percent. Maness is a distant third, with the other candidates barely registering.

The debate was only the second to include all three of the top three candidates. Maness said he would debate as often as possible, while Landrieu agreed to five televised debates. One of those five, in Monroe, was canceled when Cassidy declined to appear; two others, both in New Orleans, went ahead with just Maness and Landrieu participating.

Landrieu took a swipe at Cassidy in her opening comments. “Col. Maness, thank you for showing up again,” she said, “and Congressman Cassidy, it’s good to see you facing the voters.”

Cassidy’s cautious approach to the debates reflects his overall campaign strategy, one more typical of an incumbent than a challenger. As the best-financed Republican candidate, running with the backing of the party establishment, he looks to parlay the state’s increasing Republican tilt and the widespread dissatisfaction with Obama into a victory.

But with Maness drawing significant support and splitting the Republican primary vote, Cassidy’s chances for an outright win Tuesday are slim. Much better, according to the polls, is the outlook for him in a head-to-head matchup with Landrieu, the only remaining Democrat elected statewide. Cassidy, then, need finish no better than second Tuesday.

The Landrieu campaign initially focused on an outright win Tuesday, to avoid a straight Democrat vs. Republican runoff likely to draw intense national interest and outside money: Landrieu is a prime target of the Republicans’ drive to gain the six seats they need for a Senate majority. But she has never managed to breach the 50 percent mark in the polls, and recently she has talked more openly about the eventuality of a runoff.

Maness, 52, has never run for elective office before. A retired Air Force colonel, Maness has put together a credible campaign based on a hard-right, tea party agenda. He has raised $2.3 million for his run, compared to Landrieu’s $16.2 million and Cassidy’s $11 million.

Landrieu, 58, has never captured more than 52 percent of the vote in her three previous Senate campaigns. She defeated Republican John Kennedy with that share of the total in the 2008 open primary. In 2002, she was forced into a runoff with Republican Suzanne Haik Terrell, whom she beat 52-48. In her first run for the Senate, in 1996, she won by less than 6,000 votes, out of more than 1.7 million cast, in a showdown with Republican Woody Jenkins.

Landrieu, the daughter of a former mayor of New Orleans and the sister of the current one, was elected to the Legislature in 1979, serving two terms in the House before she won an election for state treasurer in 1987. After two terms as treasurer, she ran unsuccessfully for governor in 1995.

Cassidy, 57, practiced medicine as a doctor in Baton Rouge before winning a state Senate seat in 2006. He was elected to the U.S. House in 2008 and is in his third term in Congress.

The American Conservative Union ranks Cassidy as more conservative than the average Republican member of the House, but Maness rests further to the right on the ideological spectrum. Maness calls himself the only “true conservative” in the campaign, and he derides both Cassidy and Landrieu as creatures of Washington. Landrieu scores among the most conservative Democrats in the Senate.