Sen.Mary Landrieu (D-La.) arrives for a closed conference meeting to conduct leadership elections for the next Congress on Capitol Hill on Nov. 13. (Joshua Roberts/Reuters)

Today, the House of Representatives is expected to hold the first of two votes (the Senate may vote next week) to try to force approval of the controversial Keystone XL pipeline — a display that is widely viewed as emerging from a last-ditch political effort to improve Mary Landrieu's chances in the Louisiana Senate runoff.

But as a legal matter, it is unclear whether Congress actually has the power to ensure the pipeline gets built. There are two separate issues: Whether the executive branch (and more specifically, the State Department) does or does not retain control over giving a permit for the pipeline, since it would cross an international border with Canada; and whether states retain the power to decide on the siting of pipelines. In Nebraska, there's a lawsuit before the state Supreme Court over whether the state legislature had the power to put pipeline decisions in the hands of the governor, rather than the state Public Utilities Commission.

So can a bill out of Congress get around either of these issues?

The House bill, introduced by Landrieu's Louisiana challenger Bill Cassidy, simply says that TransCanada "may construct, connect, operate, and maintain the pipeline and cross-border facilities" described in its permit application to the State Department. The White House has said that the president takes a "dim view" of such an apparent attempt to go around the State Department's decision-making process. In general, the State Department's power to permit projects, like Keystone, that cross national borders has been deemed part of the general conduct of U.S. foreign policy.

However, in a 2012 report, the Congressional Research Service (CRS) looked at precisely this legal issue, in relation to prior Keystone bills in Congress. And it argued that "if Congress chose to assert its authority in the area of border crossing facilities, this would likely be considered within its Constitutionally enumerated authority to regulate foreign commerce." CRS argued that while lack of congressional action in the past has allowed the executive branch to decide on permitting of such so-called "border crossing facilities," congressional action could trump this. So it does look like Congress has the power to make the determination it is trying to make on Keystone.

Of course, the president also has veto power, which is his principal way to fight Congress on this matter.

And then, there's the matter of the states.

In an early 2014 decision, Nebraska District Court Judge Stephanie Stacy ruled that a 2012 Nebraska law, allowing the state governor to approve pipeline routes — instead of the state Public Service Commission — was unconstitutional. There is now an appeal of this decision pending before the Nebraska Supreme Court. So the fate of Keystone in Nebraska is very much undetermined, as the hotly contested pipeline might face challenges if back under control of the Public Service Commission. "To assume that the state level approvals are a sure thing is not accurate," says Sierra Club attorney Doug Hayes, who notes that the pipeline also needs approval in South Dakota.

Can Congress change any of this? The legislation introduced by Cassidy this week says that the pipeline may be built as proposed to the State Department, "including any subsequent revision to the pipeline route within the State of Nebraska required or authorized by the State of Nebraska."

"Whatever Congress does in its political theater would not make any difference here," says Nebraska attorney Dave Domina, who is the chief lawyer in the so-far successful lawsuit challenging the Nebraska legislature's attempt to give the governor power over pipeline decisions (Domina was also the unsuccessful 2014 Nebraska Democratic Senate contender). The CRS agrees that states can probably regulate the siting of pipelines out of "legitimate state interests in public health and safety" and not run afoul of what is called the dormant Commerce Clause of the Constitution, which has long been taken to give Congress the power to regulate interstate commerce.

However, if a state blocks a pipeline entirely, and the issue was politically significant enough, that might be another matter. Congress might then try to assert its power and “preempt” that state’s power under the Supremacy Clause of the Constitution.

So in sum: It looks like a law requiring the approval of Keystone would indeed have considerable power — and the stakes are high enough that the president might, indeed, be up for trying out his only rarely used veto power.