For all the money spent on the November elections, control of the Senate might not be decided until a Saturday three weeks before Christmas.  

Sen. Mary L. Landrieu, D-La., hopes to win re-election outright on Nov. 4 in a jungle primary against a handful of challengers. But winning a majority of the vote in a multi-candidate field would be a significant feat, and the campaigns of both Landrieu and her leading Republican opponent, Rep. Bill Cassidy, are undoubtedly preparing for an extended, one-on-one race.  

If a Dec. 6 runoff coincides with a 50-49 Republican advantage in the Senate, consultants in and out of the state warn of an unprecedented onslaught of spending from party committees and outside groups in a race that could become more about the national parties than the two candidates on the ballot.  

“Mary kind of becomes a pawn in a much, much bigger game,” said Dane Strother, a Democratic media consultant and Baton Rouge native who’s worked on previous Landrieu campaigns. “The entire force of national politics will land on Louisiana. They’ll buy every radio ad, every TV ad, inundate with direct mail. It will be a war.” Republicans like their odds in a runoff — especially if the majority is on the line — but Landrieu has won in overtime before. The Democrat won her first two of three elections for the Senate after advancing beyond a jungle primary. In her first victory, in 1996, Landrieu finished second in the primary, and the Democratic candidates on the all-party ballot totaled just 44 percent.  

But if the majority is still on the line after Election Day, the race could transform into a referendum on President Barack Obama and Senate Majority Leader Harry Reid. For all the focus and money Landrieu has spent highlighting her influence in the Senate and on catchy and clutter-cutting ads featuring her father — former New Orleans Mayor Moon Landrieu — to win the race in November, those could inevitably become secondary thoughts for voters deciding the balance of power in Washington.  

If she won in this scenario, Vice President Joseph R. Biden Jr. would be the deciding vote in an evenly divided Senate. If she lost, Republicans would take a 51-49 majority.  

“The runoff scenario where control of the Senate is on the line is the nightmare scenario for Mary Landrieu,” said Timmy Teepell, a Baton Rouge-based GOP consultant to Louisiana Gov. Bobby Jindal. “If you look at the polling numbers, and how Louisiana feels about the direction of the Obama administration, and the direction Harry Reid has taken the U.S. Senate, they’re overwhelmingly opposed to it.”  

Having the majority decided in Louisiana is possible, but countless variables would have to fall a certain way for it to happen. Republicans, who now control 45 Senate seats, would have to net five seats on Election Day. That would include building a coalition from a collection of pickup opportunities in South Dakota, West Virginia, Montana, Alaska, North Carolina, Iowa, Colorado and beyond.  

Republicans are also defending seats in Kentucky, Georgia and, if state Sen. Chris McDaniel knocks off Sen. Thad Cochran in the GOP primary runoff next week, Mississippi.  

It’s unlikely, but there might also be a runoff in Georgia, where Democrats hope to pick up an open seat. In that case, the Senate majority may not be decided until Jan. 6. Republicans would be heavy favorites to hold the seat in a runoff in the GOP-leaning state.  

Most importantly for this scenario to unfold in Louisiana, Republicans would need to hold Landrieu to 50 percent or less in the jungle primary. That will be easier for Cassidy, the favored candidate of national Republicans, with other candidates in the race. The most notable among them is Rob Maness, a retired Air Force colonel backed by Sarah Palin and the Senate Conservatives Fund. The filing deadline isn’t until Aug. 22, so the exact number of candidates won’t be known for two more months.  

Still, Republicans aren’t taking Landrieu’s political operation lightly. As one former Louisiana GOP operative said, “Nobody is going to out-campaign her on the ground.”  

In the meantime, Landrieu is working to avoid a challenging runoff in a state Obama took just 41 percent of the vote in 2012. She had $7.5 million in cash on hand at the end of March — to Cassidy’s $5 million — just before she kicked off more than $2 million in ads over about three months.  

Three of her most recent ads have featured her father, who, sitting casually alongside the senator, helps explain what she did for the state after Hurricane Katrina and issues she’s fighting the administration on, such as the approval of the Keystone XL pipeline.  

She’s put an emphasis on highlighting her influence in the Democrat-controlled chamber. As chairwoman of the Senate Energy and Natural Resources Committee, Landrieu scheduled a vote Wednesday on a bill approving the pipeline. And as chairwoman of the Senate Homeland Security Appropriations Subcommittee, she announced last week new efforts surrounding flood insurance issues.  

But the Cassidy campaign is tying her to national Democrats at every chance it gets. In an email to supporters on Tuesday, the campaign highlighted Sen. Cory Booker, D-N.J., headlining a Landrieu fundraiser in New Orleans last weekend, saying he “isn’t mainstream Louisiana.”  

Bernie Pinsonat, a Louisiana-based pollster who has worked for candidates in both parties, said to not expect to see either candidate conserve money for the runoff, especially Landrieu. And if the outcome of the runoff can give Republicans the Senate majority in 2015 — rather than simply decrease the Democrats’ majority — he doesn’t see a path to victory for her.  

“If this race is to decide who controls the Senate,” Pinsonat said, “good luck, Mary Landrieu, because Louisiana is such a red state now she won’t go back.”