U.S. lawmakers have been almost unanimous in backing both tightening sanctions on Russia and taking out of the White House’s hands the ability to lift sanctions. To be put into effect, their efforts now just await the signature of President Donald Trump—the man Russia hoped would drop the sanctions altogether.

Can the president still choose to scrap this bill? He certainly has veto power, and his government has suggested he may use it. In fact, his secretary of state, Rex Tillerson, was among the first to make the case, before a key vote on the sanctions bill in June, that the U.S. government needs to go into discussions with Russia and any other government with “flexibility” on matters such as sanctions—something that secretaries of state would be stripped of if White House oversight is replaced by congressional oversight.

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“There’s a potential gulf between signing a law and actually implementing it,” says Pamela S. Karlan, professor at the Stanford School of Law. “One frequent historical example is a president who signs a law because it contains provisions that are critical to keeping the government running, but who announces at the time he signs the law that he considers particular provisions in the law unconstitutional and that he will not abide by them.”

Karlan cites an example from 2002: Under President George W. Bush, Congress passed a bill that required the secretary of state to put the word “Israel” on the passport of an American citizen born in Jerusalem, if that citizen requested it. However, the legal status of that holy city is one of the issues at the heart of the Israeli-Palestinian conflict—according to the U.N., the city is a separate entity (corpus separatum), and the U.S. State Department therefore does not recognize it as part of Israel. Official U.S. documents such as a passport list the location of the city simply as “Jerusalem” without specifying a country.

According to Karlan, the provision “was part of an appropriations act that the president needed to sign to keep the government running.

“When President Bush signed the act, he announced that he considered that requirement unconstitutional because it trenched upon the president’s foreign affairs power. When the parents of a boy born in Jerusalem requested that the word ‘Israel’ be put on his passport, the State Department refused,” she says.”