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 ISIS 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, Cassidy 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 ISIS.

The debate took place in the Manship School of Mass Communication at LSU. It was broadcast statewide.

None of the candidates stumbled badly in the debate. All reinforced their basic messages, and likely solidified their appeals to their core supporters. Afterward, the Twittersphere was filled with claims of victory from each candidate’s camp.

Not surprisingly, the Affordable Care Act 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,” he 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 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, who claims the tea-party label, 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 to set the stage for a desired showdown with Landrieu.

The Landrieu campaign initially focused on victory 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.

Landrieu is seeking a fourth six-year term in the Senate. Due to her seniority, she took over early this year as chair of the Senate Energy and Natural Resources Committee, with oversight of the oil and gas industry that is central to the state’s economy. She has frequently touted her “clout” in the Senate and her ability to deliver for Louisiana interests.

“Sen. Landrieu has clout,” Cassidy said, “but she uses it for Barack Obama.

“If you love Barack Obama’s agenda, vote for Sen. Landrieu.”

Maness, 52, has never run for elective office before. A retired Air Force colonel who lives in Madisonville, Maness has raised $2.3 million for his run, compared to Landrieu’s $16.2 million and Cassidy’s $11 million.

The daughter of a former mayor of New Orleans and the sister of the current one, Landrieu, 58, was elected to the state House in 1979 and as state treasurer in 1987 before moving to Congress.

Cassidy, 57, practiced medicine in Baton Rouge before winning a state Senate seat in 2006. He was elected to the U.S. House in 2008.

The American Conservative Union ranks Cassidy as more conservative than the average Republican member of the House. Landrieu scores among the most conservative Democrats in the Senate.

Elizabeth Crisp of The Advocate Capitol news bureau contributed to this report.