Karen Heller/Washington Post

NEW ORLEANS --  Only one big number matters in Republican Rep. Bill Cassidy's challenge to three-term Democrat Sen. Mary Landrieu: 97, the percentage of times he says Landrieu voted with President Barack Obama, hugely unpopular in populist Louisiana.

In the first few minutes of conversation with the Baton Rouge physician as he visited Landrieu country Saturday, he managed to mention 97 percent and Obama three times -- and he was just getting warmed up. (Of course, Republican Gov. Bobby Jindal is even less popular, akin to Alabama football, and has been persona non grata in this campaign.)

But another number also matters in Louisiana's jungle primary, where there are nine candidate and three top contenders: 50 percent.

If no candidate garners half of all votes Tuesday,  the top two contenders -- most likely Landrieu and Cassidy -- will campaign for another four hugely expensive weeks in a Dec. 6 runoff, making this the Amazing Race of U.S. Senate campaigns.

Cassidy, a professed optimist who quoted Scripture, Aristotle and C.S. Lewis in a single interview, has no illusions: "I think there's going to be a runoff."

Landrieu, whose brother is the well-liked mayor of this city, as was her father, has a scheduled rally with Hillary Clinton Saturday afternoon -- she wouldn't dare ask the president to visit Louisiana, even in the Democratic stronghold of New Orleans. Cassidy met with retired neurosurgeon and conservative icon Ben Carson: "I think he's more attractive, head to head, as a presidential candidate."

Cassidy has been painted as drier than dry rub by journalists. A Politico headline asked "Is Bill Cassidy too boring for Louisiana?" Not that Landrieu is a wild character, though she did aid in a keg stand and do the "Wobble" dance at two separate football games thereby possibly winning the tailgate vote.

But here's the thing: The longer you spend with Cassidy in the final days of Louisiana's endless, brutal and anything-but-soon-over Senate race, and the sooner he goes off message and his mantra of 97 percent, the looser he gets.

On Saturday, waiting for a Breast Cancer Walk at Champions Square in the shadow of the Superdome (which might as well be called St. Drew Cathedral), the three-term Congressman stretched, danced and sported a pink jersey and pom pom. He was, at times, agitated and impassioned.

Cassidy has been running for the Senate since April 2013. Ads have been on television since Christmas (happy holidays!), and most have been relentlessly negative. They now air virtually non-stop, so much that viewers may long for mattress and used-car commercials. In one single, brutal week in September, not a single positive campaign ad ran in the state.

"When we started advertising, we were all positive. It was a beautiful ad," Cassidy said, stretching his leg, "and we were getting killed."

Tea Party candidate and retired Air Force colonel Rob Maness, who has the blessing of Sarah Palin and Louisianan Tony Perkins of the Family Research Council, is Cassidy's expensive headache and spoiler in this jungle primary, polling at as high as 13 percent.

Without Maness on the ballot, Cassidy leads Landrieu in every poll.

"What can you say? I can't curse the wind, " said Cassidy. "You do what you have to do."

Which, for Cassidy, means not mentioning the Tea Party candidate by name. Maness is the Voldemort of his campaign.

Instead, Cassidy said most emphatically: "I'm running against Mary Landrieu."

Cassidy has run a smart race, often acting like the incumbent. He agreed to only two debates, the final one Wednesday. Landrieu greeted him by saying,"Congressman Cassidy, it's good to see you finally facing the voters for this hour."

The congressman dismissed the debates as largely a waste of time. "Wasn't much of that you heard redundant? You get one minute to express your views. I'd rather meet with voters."

Much has been made, mostly by the opposition, of Cassidy's previous support for Democratic candidates, including writing a check for a prior Landrieu campaign.

"Look, I gave her some money because someone asked me for some money for a wine tasting," he said Saturday. "Call it a youthful indiscretion."