With little more than a week left, a cavalry of conservative groups has rallied to Cassidy's side, spending millions on the airwaves to help him close strong. Meanwhile, a parade of well-known Republicans from around the country has joined the physician-turned-congressman on the campaign trail.
The bustling GOP bandwagon stands in stark contrast to the Democratic side, where the groups that usually send in reinforcements have declined to spend money for Landrieu, sensing virtually no chance of shaking up a race that looks like Cassidy's for the taking.
"Unless he makes a big mistake, says something stupid or some scandal erupts," Cassidy should win, said University of Louisiana at Monroe political scientist Joshua Stockley.
If he does, it would add to Democratic woes in a region they once dominated and continued to lose ground in this year. Rep. John Barrow (D-Ga.), the last white Democrat from the Deep South in the House, lost his seat on Election Day. Arkansas, once a Democratic stronghold, will feature two Republican senators and a GOP governor. Meanwhile Landrieu is her party's most powerful figure left in Louisiana.
A Washington Post review of television and radio advertising spending in the runoff shows that between Nov. 5 and Dec. 6, pro-Cassidy forces are set to dish out more than $5 million on advertising -- about 100 times what Landrieu's allies are on pace to spend. The outside groups helping Cassidy include the National Republican Senatorial Committee, which is on pace to spend more than $1.4 million ads, records show.
Still reeling from the stinging defeats of Nov. 4, the Democratic Senatorial Campaign Committee pulled all of its TV ad reservations. Senate Majority PAC, a leading Democratic super PAC, has had no air presence in the runoff. The only group on the air for Landrieu: The Democratic State Central Committee, which spent about 53,000 on radio advertising.
Landrieu is set to spent about $1.4 million on advertising in the runoff, behind Cassidy's projected $2 million investment.
Faced with the prospect of a low-turnout runoff dominated by conservative voters, Landrieu is trying to cobble together a surprise comeback in the face of a series of setbacks.
The GOP takeover of the Senate means Landrieu will lose her clout as chairman of the powerful Senate Energy Committee, a position she touted as a centerpiece of her reelection pitch leading up to Nov. 4. Making matters worse, Republican leaders have promised Cassidy a seat on the committee, which is closely-watched in oil-rich Louisiana.
The Democrat tried to show she still has some muscle by spearheading a push to compel President Obama to construct the Keystone XL Pipeline. But that effort fell short in the Senate by a single vote last week with most of Landrieu's own party bucking her.
Landrieu will get some fundraising help next week from Hillary Clinton. And some of her colleagues, including Sens. Joe Donnelly (D-Ind.) and Cory Booker (D-N.J.), have campaigned for her in hopes of boosting turnout among moderate Democrats and African Americans.
But Cassidy has countered by campaigning alongside marquee names like Sens. Rand Paul (R-Ky.) and Marco Rubio (R-Fla.), Ben Carson and Texas Gov. Rick Perry (R) -- all potential White House hopefuls. Observers anticipate a right-leaning electorate to show up to vote in the runoff, including those who count themselves as supporters of the national GOP stars.
In the all-party primary, about 52 percent of voters -- some 1.5 million -- turned out to vote. Landrieu won 42 percent of the vote, while Cassidy won 41 percent. Tea party Republican Rob Maness, who has since endorsed Cassidy, claimed 14 percent of the vote.
In other words, Republican candidates won a majority. Meanwhile pre-primary polls showed that in a runoff, Cassidy would have an advantage over Landrieu.
Black voters made up about a third of the Nov. 4 electorate and voted overwhelmingly (94 percent) for Landrieu, according to exit poll data, which has spurred Landrieu's campaign to try to urge them to come out in large numbers on Dec. 6. But Landrieu won only 18 percent of the white vote, which made up two-thirds of the electorate.
"The electorate is going to be older, whiter, better-educated and wealthier than what we saw on [Nov. 4]," said Stockley, who predicted runoff turnout would not eclipse 40 percent.
Landrieu's stretch-run strategy is to convince voters that she has had their back, while Cassidy does not. A recent ad casts sought to cast Cassidy as a rambling figure who can't be trusted to preserve Social Security.
"Mary Landrieu is always there fighting for them, delivering for them," said the senator's campaign spokesman, Fabien Levy.
Cassidy and his allies, meanwhile, have kept up their long-standing efforts to tie Landrieu to Obama and his policies. It's been a potent line of attack in a state where the president is unpopular.
The conservative super PAC Conservative War Chest unleashed a two-minute pro-Cassidy Thanksgiving attack ad assailing Obama.Only at the very end does it mention Cassidy.
"Senator Landrieu's record of supporting President Obama's record 97 percent of the time is a rallying cry for people who want change in D.C.," said Cassidy spokesman John Cummins.
Landrieu has fared well in runoffs before, winning in 1996 and 2002. But the tough environment that felled many of her colleagues may be about to claim her, too.
"Clearly, the national context and Louisiana context is favorable to Republicans this time around," said Stockley.