Senate elections 101: Louisiana is a referendum on Mary Landrieu.

The Monitor's "Senate elections 101" series looks at the specific issues that will be driving voters in each of the 10 tossup races that will determine who wins control of the Senate.

In Louisiana, they’re called the Big Three: energy, military bases, and disaster relief. If three-term Sen. Mary Landrieu (D) is to win reelection, those are the issues that will save her.

Oil and gas are the state’s top industry. The military is also a top employer, and the battle to save bases has been hard fought. The aftermath of hurricane Katrina needs no introduction. Overarching all these issues is Senator Landrieu’s claim that her seniority – foremost, as chair of the Senate Committee on Energy and Natural Resources – gives Louisiana clout in Washington that her main opponent, Rep. Bill Cassidy (R), wouldn’t have.

What she doesn’t say is that for her to remain committee chair, the Democrats will need to hold their Senate majority – not a popular idea in deep-red Louisiana. And besides, Landrieu’s critics say, the popular Keystone XL pipeline still hasn’t been approved. Where’s her influence? they ask.

Congressman Cassidy has played the race low-key, turning it into a referendum on Landrieu – and by extension, President Obama. But unlike some Republican candidates, Cassidy hasn’t shied away from getting specific about the Affordable Care Act.

Cassidy, a physician, says he believes access to health care is a right – not a standard GOP line. He would “repeal and replace” the ACA, or Obamacare, but wants to find a way for people with preexisting conditions to remain covered. In a debate, he also didn’t back down from his controversial view that the Social Security retirement age should be raised to 70. And he supports medical marijuana, which Landrieu does not.

Will any of these issue positions matter? That’s not clear.

“I’m always a little bit skeptical of this notion that an issue comes along and voters make up their minds based on that issue,” says Michael Henderson, a political scientist at Louisiana State University in Baton Rouge.

Bigger factors are more important, he says, such as a voter’s ideology and how voters feel the country is doing. “I would say that it’s still about the president,” Mr. Henderson says.

Professor Henderson’s poll of the Louisiana electorate in September ranked the economy and foreign affairs as voters’ top concern. The economy and jobs are almost always No. 1 with voters, elements that feed into a larger sense of how the country is doing. Foreign affairs likely ranked high, Henderson says, because of the rise of the Islamic State, the beheadings of two Americans, and Mr. Obama’s authorization of air strikes in Syria, which happened while the LSU poll was in the field. Ebola is another issue that makes voters feel insecure.

Even if Landrieu comes in first on Election Day, the race may not be over. If no one wins a majority, the top two finishers will face a runoff vote on Dec. 6. At that point, voters for the tea party candidate, Rob Maness, are likely to favor Cassidy.  So for Landrieu to get reelected, her best bet is to win outright on Nov. 4.

Please read our other entries in this series:

• Louisiana is a referendum on Mary Landrieu
• Colorado could come down to women's issues
• Kentucky conflicted about Mitch McConnell
• Iowa split between two very different candidates
• Georgia could turn on a David Purdue gaffe
• North Carolina wary of Tillis's tea party revolution
• Arkansas considers ending its blue-state legacy
• Alaska's remotest places could be crucial
• New Hampshire shapes up as carpetbagger vs. rubber stamp 
• The big Kansas issue Pat Roberts isn't talking about