Clout.

U.S. Sen. Mary Landrieu, the Democratic incumbent from Louisiana, says she’s got it. The Republican challengers to her re-election say she doesn’t.

Both sides are right.

The clout they are talking about is the ability to wield influence in the Senate to advance Louisiana’s interests.

Landrieu says her three terms in office have built her clout in a Senate hierarchy that rewards seniority. The big payoff came earlier this year, when she was named to chair the Senate Energy and Natural Resources Committee, filling a vacancy created after the departure from Congress of a longer-serving senator.

Committee chairs exercise considerable authority over the legislation referred to their committees. They decide when a bill will come up for committee consideration — or if it will come up at all. And virtually all legislation must win a majority vote in a committee to advance to the Senate floor for final approval.

The jurisdiction of Landrieu’s committee includes the oil and gas industry central to her state’s economy. Landrieu is as friendly to that industry as any Democrat in the Senate, in sharp contrast to her predecessor as chair, Ron Wyden, of Oregon.

So it figures Landrieu can get more pro-industry legislation over the crucial hurdle of committee approval — but that’s not the end of the race. And it’s in the next lap — on the Senate floor — where Republicans say her clout fades to insignificance.

In the 100-seat U.S. Senate, the majority party sets the agenda, picks committee chairs and makes up a majority of each committee. Right now, that’s the Democrats (although their 55-member majority includes two independents who vote with them on procedural matters). The Senate majority leader — currently Democrat Harry Reid, of Nevada — plays a role on the floor like the chair of a committee, deciding if and when bills are put to a vote. But if he wants to keep his job, he needs to please a majority of the Democrats — and most of them lack Landrieu’s affection for oil and gas companies.

That means energy bills Landrieu favors may be sidetracked by Reid. It’s the nature of the system — the same system that put Landrieu in the chair of the energy committee in the first place.

The Keystone pipeline is a case in point, and it’s been brandished by both sides in the Senate campaign. At issue is completion of the final, northern section of the pipeline, which would link Canadian tar-sands oilfields to refineries on the Texas Gulf Coast. Supporters of the project — who include Landrieu and her two main Republican challengers, U.S. Rep. Bill Cassidy, of Baton Rouge, and retired Air Force Col. Rob Maness, of Madisonville — say it would create jobs and strengthen the nation’s energy infrastructure. Opponents say it represents a step backward in terms of reducing carbon emissions, especially as it would transport relatively “dirty” oil from the tar sands.

Because the pipeline crosses an international border, the power to approve or reject it rests with Democratic President Barack Obama, who has put off a decision for years. But Landrieu co-sponsored a bill with a Republican senator to override the White House and clear the way for the pipeline. In June, she brought the bill before her committee, and she lined up with a fellow energy-friendly Democrat and the committee’s Republicans in a 12-10 vote to send the legislation to the Senate floor. It has not been seen since.

Cassidy and Maness argue that the solution is to replace Reid with a Republican by giving the Republicans a Senate majority — and Landrieu is a primary target of the Republican drive this fall to pick up the six seats they need to take over.

But, as Reid and the Democrats have discovered, it’s not that simple.

Senate rules on limiting floor debates effectively raise the bar to 60 votes as necessary to pass bills, and not even the most optimistic Republican scenarios see the party approaching that mark. Although several Democrats support Keystone, that still might not be enough to send the measure to the White House — where, in any case, Obama could veto it. It takes 67 votes in the Senate to override a veto.

Gregory Roberts is chief of The Advocate Washington bureau. His email address is groberts@theadvocate.com. Follow him on Twitter, @GregRobertsDC.