The 140-character limit was more than enough for Bill Cassidy to capture the essence of his U.S. Senate campaign in a tweet from his Twitter account last week:

“I’m proud to oppose President Obama 92% of the time, while Senator Landrieu supports him 97 percent of the time.”

No need for Cassidy to discuss his record during his three terms in the U.S. House representing Baton Rouge, nor his position on specific issues, nor his medical career as a gastroenterologist and hepatologist. It’s enough, in terms of Cassidy’s central campaign strategy, to declare his opposition to Barack Obama and his opponent’s fealty to Obama. The test — although likely not the final test — of that strategy comes Tuesday, with the open primary for the Senate and other offices.

Cassidy’s game plan is not much different from strategies Republicans are applying in states across the country in a drive to pick up the six seats they need to wrest the Senate majority from the Democrats. Control of the Senate is the big national prize in this nonpresidential election year, and Republicans everywhere are doubling down on the unpopularity of the Democratic president.

Mary Landrieu looms large and vulnerable in the Republicans’ sights as they seek the upper hand in the Senate. Landrieu’s 2008 race, when she was elected to her third term in the Senate, was the last time a Democrat won a statewide office in Louisiana. As for Obama, he lost badly in Louisiana both that year and in 2012; both times, three of five voters favored his Republican opponent. His popularity in the state remains stuck at close to that level, according to an opinion survey released Thursday by the University of New Orleans.

Cassidy’s one-note song apparently is playing pretty well with voters. With a second Republican — retired Air Force Col. Rob Maness, of Madisonville — drawing support from many tea party faithful, Cassidy is well short of the majority needed to win election outright Tuesday, according to the UNO poll and most others in the race. But Landrieu, too, is short of 50 percent and seems destined to meet Cassidy in a Dec. 6 runoff between the top two primary finishers. In that theoretical Democrat vs. Republican matchup, her lead over Cassidy in the primary field vanishes, and he wins by a clear margin, the UNO poll and others predict.

With both the current political trend and the underlying numbers in his favor, Cassidy, 57, has run a cautious campaign, less typical of a challenger than of an established incumbent seeking to avoid mistakes (and also, it seems, in keeping with his personality: He comes across as careful, reserved, somewhat stiff). His TV appearance Wednesday night with Landrieu and Cassidy was just the second time he’s agreed to debate them on camera (Landrieu and Maness participated in two other TV debates that Cassidy spurned; a fifth was canceled because he refused to show up).

When he has debated them, Cassidy has followed his favorite tune. “If you love Barack Obama’s agenda, vote for Sen. Landrieu,” he said at one point Wednesday night. At another, he asked of Landrieu, “My gosh, why does she support the fellow?”

For her part, Landrieu, 58, has very much talked up her accomplishments in the Senate and, in particular, her record of “fighting” (her favorite verb) for the interests of Louisiana residents. Her tweets from the past week tell a story of their own: There she is in Houma, speaking about a $500,000 federal security grant she helped deliver to the Port of Terrebonne; and again, in Franklin, talking about how she kept her promise after Hurricane Gustav and came through with a new flood-protection gate; and again, in Henderson, celebrating her success in pushing for a $19 million federal payment to crawfish processors to compensate for illegal market “dumping” by Chinese importers. It’s old-school, bring-home-the-bacon politics.

Landrieu grew up in a political family — her father was mayor of New Orleans while she was in high school and college, and her brother is mayor now — and has spent virtually her entire adult life in elective office. She’s a classic happy warrior on the campaign trail, this year going viral on the Internet, once for assisting at a keg stand at an LSU tailgating party and again for joining in a “wobble” line dance at a Southern University tailgate.

She’s faced challenging elections before and has never won more than 52 percent of the vote. In her first Senate run, in 1996, she finished second to Republican Woody Jenkins in the primary and then beat him in the runoff by 5,788 votes — a number her staff has heard her repeat so often they, too, have it memorized.

In 2002, she was forced into a runoff with Republican Suzanne Haik Terrell. Only in her most recent election, in 2008, did she win in the primary.

But this year, the odds seem especially long.

“I think the overall environment is much more toxic for her,” Southern University political science professor Albert Samuels said. “In 2008, when she was running for a third term, there was a lot of excitement among Democrats about electing an African-American president. The momentum was on the side of the Democrats.”

This year, Republican voters register more energy and motivation in opinion polls, driven by anti-Obama fervor.

Charlie Cook, of the national Cook Political Report in Washington, D.C., likens the situation of Landrieu and other vulnerable Senate Democrats to that of an ocean swimmer who may have Olympic-level skills yet can still be overwhelmed by a strong undertow.

“When you have a president whose popularity is way down, it could be pretty treacherous,” he said. “Let’s face it: The Democratic Party in the South is really under siege.”

Joshua Stockley, a political scientist at the University of Louisiana at Monroe, said, “The political climate in Louisiana is incredibly significant — and it’s the largest driver to explaining why Sen. Landrieu is in the fight of her life.”

But pollsters and pundits don’t decide elections; voters do — specifically, the ones who make the effort to fill out a ballot at the polls.

Who those voters are, and how many, can fluctuate dramatically from election to election. A strong voter turnout tends to benefit Democratic candidates, because some of their traditional constituencies — minorities, young voters, lower-income voters — do not participate in elections as regularly as older, wealthier white voters, who lean Republican. Turnout peaks in presidential election years, when interest is high.

To counter those forces, the Democratic Party is spending $60 million in Senate races in 10 states, including Louisiana, in a thoroughgoing get-out-the-vote program that involves field offices, volunteers, electronic data banks and the results of academic research. Modeled after Obama’s state-of-the-art turnout operations in presidential battleground states in 2008 and 2012, the undertaking brings to Louisiana an unprecedented level of intensity and organization in mobilizing occasional Democratic voters.

“Sen. Landrieu needs Democrats to show up and vote,” Stockley said, “and most importantly, she’s really got to get African-Americans out to vote.” Just over 30 percent of registered voters in Louisiana are African-Americans, and they have provided Landrieu near-unanimous support in past elections.

If successful, the Democrats’ expensive initiative could be the card that allows Landrieu to fill an inside straight. But Republicans, too, paid attention to what happened in 2008 and 2012, and they, too, have geared up a bigger turnout effort than they’ve ever produced before in Louisiana — if still representing less of a financial investment than their Democratic counterparts.

Will the ramped-up Democratic ground game be enough on a tilted playing field?

Some results are available already, from the Oct. 21-28 early voting period that allowed voters to fill out their primary ballots in advance of Nov. 4. Overall, the numbers show a higher level of early voter participation than in 2010 — a nonpresidential year when the state’s other Senate seat was on the ballot — but lower than in 2008 or 2012.

But the early voting data are subject to interpretation and to spinning by both sides.

Maness, 52, shapes up as something of a wild card in the primary, or maybe, for Cassidy, a spoiler. He’s never run for elective office before, spending 33 years in the Air Force before retiring in 2011. Although posted for a few years to Barksdale Air Force Base near Shreveport, he did not come to Louisiana for good until he left the service, when he moved to Madisonville.

Maness runs to the right of Cassidy, who himself is rated more conservative than the average House Republican by the American Conservative Union (Landrieu ranks among the most conservative Senate Democrats). Maness has raised $2.3 million, which is considerably less than either Landrieu or Cassidy, and he lacks the kind of field operations supporting them. He polls at around 10 percent — 20-plus points behind the front-runners — and if he makes the runoff, it would amount to a major surprise.

Follow Gregory Roberts of The Advocate Washington bureau on Twitter, @GregRobertsDC.