— If U.S. Sen. Mary Landrieu wants to figure out how to win re-election as a Democrat this year in a state where the political current runs strongly in favor of Republicans, she can examine the voting results from November 2008 — the last time she won a Senate race in Louisiana.

In 2008, the Democrat at the top of the ticket — Barack Obama, who would win election as president — took just 40 percent of the vote in Louisiana against Republican candidate John McCain. But Landrieu captured 52 percent of the vote to beat her Republican opponent, John Kennedy.

Landrieu’s problem is that it’s not 2008 anymore. Her principal Republican opponent this time is a three-term Baton Rouge congressman, Bill Cassidy, who is well-financed and a key player in the nationwide Republican effort to pick up the six seats the party needs to seize a majority in the Senate (the other significant Republican candidate, retired Air Force Col. Rob Maness, is more of an obstacle for Cassidy than for Landrieu). That national landscape could make Landrieu’s task especially daunting if, as polls suggest, she falls short of the majority vote needed to win outright on Nov. 4 and is forced into a head-to-head runoff with Cassidy on Dec. 6.

Maybe more importantly, 2014 is not a presidential election year but falls instead in the middle of Obama’s second term. That timing bodes ill historically for members of a president’s party running for federal office. And it means there’s no compelling presidential candidate on the ballot to motivate turnout by traditionally Democratic voters, as Obama, an African-American, achieved with African-Americans, his party’s most reliable constituency in Louisiana.

“A presidential election versus a midterm is like apples and oranges,” said Charlie Cook, who runs the national Cook Political Report in Washington. “The challenge for Democrats this year is that a lot of people will show up to vote in a presidential election, competitive (in their state) or not, but in the midterms, they don’t.”

That’s true across the Louisiana electorate: In the presidential-election years of 2008 and 2012, roughly 2 million people voted in Louisiana, but the number dropped to 1.3 million in 2010, when the election for the state’s other U.S. Senate seat led the ballot. The falloff, though, was considerably sharper among black voters.

“In a midterm election,” Cook said, “the turnout is older, it’s whiter, it’s more conservative, it’s more Republican.”

Republicans across the country also are counting on the Obama effect, seeking to link Democratic candidates to a president whose nationwide approval ratings have dipped well below the 51 percent of the electorate that gave him a second term in 2012. That’s the Republican strategy in Louisiana, too, although Obama’s current level of approval in the state isn’t much different from the approximately 40 percent of the vote he won in Louisiana in both 2008 and 2012.

“The dynamics were different,” Cook said of those earlier Louisiana elections. “The Democratic Party was unpopular but not radioactive.”

Landrieu’s potential road to victory is clear — but that doesn’t make it smooth.

“Her best path to winning the seat is to win outright in November,” said Bryan Brox, a political science professor at Tulane University in New Orleans. “Her biggest goal at this point is to try to mobilize people who normally wouldn’t turn out in a midterm election — the types of people who voted for her in 2008 but would normally take the midterm off.”

That means, first and foremost, African-American voters, who make up about 30 percent of the state’s electorate.

“There’s no doubt she’ll get the lion’s share of that vote,” said Albert Samuels, a political science professor at Southern University in Baton Rouge. “But she needs to get that vote energized and excited. Turnout is going to be critical.”

Another demographic target of opportunity for Landrieu are young voters, who tend to be more Democratic, said Pearson Cross, chairman of the political science department at the University of Louisiana at Lafayette.

“But that’s not enough,” Cross said. “If young people come out and blacks come out, that’s not enough.”

Those two groups of voters would provide Landrieu about a third of the total vote, or maybe a little more, Cross said. To get past 50 percent, he said, Landrieu needs support from white voters — something approaching the 35 percent she received in 2008, which was 21/2 times the rate of white support for Obama.

“If you drop below 30 percent of the white vote, chances are you’re not going to make it,” Cross said.

Landrieu drew the white voters she needed in 2008 by appealing to conservatives and moderates, said Joshua Stockley, a political science professor at the University of Louisiana at Monroe.

“In 2008, according to the exit polls, she attracted 30 percent of self-described conservatives,” Stockley said. “That’s a phenomenal percentage for a Democrat. She has to develop a rapport among conservatives similar to that.

“Likewise, she won the moderate vote against John Kennedy in 2008. The things I’m looking for are, what’s her support among conservatives? What’s her support among moderates?”

In 2008, Landrieu’s biggest haul of votes came from New Orleans. But that was predictable: It’s the largest city in the state, with a majority-block population, and Landrieu grew up there (her brother, Mitch, is now the city’s mayor). She barely outperformed Obama in New Orleans.

In a dozen parishes, Landrieu collected at least 5,000 more votes than Obama in 2008, meaning she benefited from white voters splitting their tickets there. With the exception of Caddo Parish, around Shreveport, and Rapides Parish, around Alexandria, those parishes are in the southern half of the state. For Landrieu, a Catholic Democrat from New Orleans, the conservative, Protestant north of the state is forbidding territory.

Landrieu’s zealous efforts to win federal assistance in the aftermath of Hurricane Katrina no doubt helped her in the southeastern Louisiana parishes among that dozen: Ascension, Jefferson, Lafourche, Livingston, St. Tammany, Tangipahoa and Terrebonne. But Katrina, which struck the state in 2005, was still a fresh memory in 2008; now, it’s nearly 10 years in the past.

For Cassidy, the path to victory seems considerably easier.

“He’s got a lot bigger margin for error than Sen. Landrieu,” Stockley said.

With Maness included in the field and drawing tea party support, polls indicate that Cassidy, too, will fall short of the majority needed for election on Nov. 4. But when matched head-to-head with Landrieu — the likely runoff pairing — Cassidy consistently bests Landrieu in the surveys.

“Basically, unless he does something stupid, he should make the runoff,” Samuels said.

The factors that would be at play in a runoff are difficult to predict: With control of the Senate up for grabs, much would depend on what happens in several other Senate battleground states on Nov. 4. If the balance hangs on the Dec. 6 Louisiana vote, both national parties will focus money and energy on the state.

But in a runoff with Landrieu, Cassidy will be on the ballot as the lone Republican in a state that regularly votes Republican. Landrieu is the last remaining Louisiana Democrat elected statewide. Voter turnout on Dec. 6 likely will be lower than on Nov. 4, which would favor Cassidy as well, Brox said.

“The electorate will be even more partisan and extremist,” Brox said, “and the people who are most motivated this time are the people who are against Mary Landrieu.”

Cassidy “needs to continue what he’s doing: making this a referendum on Landrieu and on her affiliation with the national Democratic Party,” Brox said. “Whether she’s a moderate or conservative doesn’t matter. She still has that D after her name.”

As for Maness, his chances to win election seem virtually nil. He lags well behind both Landrieu and Cassidy in the polls and in fundraising, and the Republican establishment is against him. A political newcomer, he has no traditional geographic or demographic base, and the tea party tide has been ebbing nationwide.

“Barring some serious mistake or gaffe or lack of judgment or some kind of revelation of which we’re not aware — an October surprise — it’s really hard to envision a scenario in which Maness somehow makes it into the runoff,” Samuels said.

“Maness has no path to victory in 2014,” Stockley said. “The only path he has is the path to spoiling the election for Rep. Cassidy.”

“He could win a Pyrrhic victory,” Cross said. “Anything Rob Maness gets over 10 percent of the vote is going to be a victory for him.”

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