'New decision-making structure'

Pipeline opponents say Trump acted illegally. They have asked a federal court to block the new Keystone permit, arguing that it is an effort to get around an earlier court ruling.

But one legal expert said Trump's approach might succeed.

"He has now created a whole new decision-making structure" for cross-border pipelines, said Richard Pierce, a law professor at George Washington University.

If the courts follow a 1992 Supreme Court ruling, they may find that action taken by the State Department in approving or rejecting the pipeline "is nonreviewable, because it doesn't qualify as final agency action," Pierce said. Further, Trump's decision would not be subject to review because of a separate law that declares the president is not an agency and therefore is not bound by rules that apply to agency actions.

"That's a very clever approach that might well work," Pierce said.

Trump's actions are "typical of this presidency," said Holly Doremus, an environmental law professor at the University of California-Berkeley. She said Trump frequently seeks to stretch the limits of his power, and she cited Trump's declaration of an emergency that he says allows him to shift more money to construction of a promised wall along the U.S.-Mexico border.

In the case of Keystone, Trump appears to be arguing that the new presidential permit, issued March 29, gets around restrictions under the National Environmental Policy Act or other laws, because the statutes apply to executive-branch agencies but not to the president, Doremus said.

"If the president is the only discretionary decision maker, NEPA simply does not apply," she said.

Who decides?

While Trump's theory is plausible, it is unclear who is the ultimate decision-maker on Keystone XL, Doremus said. The pipeline would ship crude oil from the tar sands of western Canada to U.S. refineries along the Gulf of Mexico.