In the signing statement, Trump wrote that he would only enforce the certification requirement to the extent it is “feasible and consistent with the President’s exclusive constitutional authorities as Commander in Chief.”

The new law requires Secretary of State Mike Pompeo to certify that the Saudis and Emiratis are reducing civilian casualties, avoiding the destruction of critical civilian infrastructure, engaging in diplomacy and ensuring the access of humanitarian goods into Yemen. Absent such a certification or a national security waiver, the Trump administration must withdraw its midair refueling support, which is essential for the Saudis and Emiratis to continue their aerial bombing campaign on behalf of Yemen’s internationally recognized government.

“Particularly in light of the horrific hit against the school bus last week, that’s very disappointing to hear the White House take that approach,” Sen. Jeanne Shaheen, D-N.H., a key architect of the Yemen language, told Al-Monitor. “We should be doing everything we can to try to reduce casualties in Yemen to try to get the parties to the table and to urge the Saudis and Emiratis to do everything they can to avoid the kind of casualties that we’re seeing.”

Shortly after Trump signed an annual defense authorization bill on Monday, the White House issued a statement objecting to several provisions in the law, including language that would make it more difficult for the United States to continue support for the Saudi-led coalition in its aerial campaign against the Iran-backed Houthi rebels.

President Donald Trump signed several restrictions on US military involvement in Yemen into law earlier this week. There’s just one caveat — Trump is threatening to ignore most of those provisions, prompting swift backlash from Democrats.

“I’d like to know what their legal rationale is for ignoring stuff we put into the [National Defense Authorization Act],” Sen. Tim Kaine, D-Va., a member of the Armed Services Committee, which drafted the bill, told Al-Monitor. “And they certainly didn’t reach out to us to tell us not to put them in, so that’s kind of weird.”

Further exacerbating the confusion, Trump’s nominee to become under secretary of state for political affairs, Ambassador David Hale, committed to upholding the Yemen certification requirements during his confirmation hearing today.

“If we’re unable to find the elements that we’re required to certify, then we will of course have to act accordingly; that’s the law,” Hale promised the Senate Foreign Relations Committee. “I’m not fully briefed on the details of this legislation, but I understand the significance of this and the need to be in accordance with the law.”

Kate Kizer, the policy director of Win Without War, a coalition of activist groups that aggressively pushed Congress to pass the Yemen language, called Trump’s signing statement “preposterous,” noting that the law overwhelmingly passed 359-54 in the House and 87-10 in the Senate.

“If Trump makes good on his threat to disobey the law, we expect there will be a bipartisan showdown in Congress …"

“If Trump makes good on his threat to disobey the law, we expect there will be a bipartisan showdown in Congress, which has already come incredibly close to cutting off US assistance to Saudi Arabia and the UAE in Yemen,” Kizer told Al-Monitor.

Indeed, Republican lawmakers were instrumental in crafting the law’s Yemen provisions. Both parties have raised alarm bells as famine and an unprecedented cholera crisis plague the country while an Emirati siege on the Houthi-controlled port of Hodeidah threatens to further restrict aid for impoverished and displaced Yemenis.

An Associated Press report this month indicating the United States is aware that the UAE has recruited al-Qaeda forces to fight against the Houthis has only added to the congressional angst. The AP also reported last year that US troops have been present alongside Emirati forces as they tortured detainees during interrogations.

Nonetheless, Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., another key architect of the law’s Yemen provisions and frequent Trump critic, is reserving judgment on the White House’s defiant statement until the administration has a chance to implement the bill.

“Every administration does these kind of things,” Corker told Al-Monitor. “They may counter because they feel like constitutionally we’re trying to take authorities that they have away, but we still have ways of pushing so that these things continue to stay in place.”

But Democrats are getting ready to play hardball.

The same day Trump issued the signing statement, House Minority Whip Steny Hoyer, D-Md., and the top Democrat on the House Armed Services Committee, Rep. Adam Smith, D-Wash., led 28 other Democrats in a request for a briefing on Yemen in September via a letter addressed to Pompeo, Defense Secretary James Mattis and National Intelligence Director Dan Coats.

Citing the bill’s Yemen certification requirements, the letter publicly stipulates several pointed, specific questions for the administration to answer in the briefing.

In a separate letter on Tuesday, Senate Armed Services Committee member Elizabeth Warren, D-Mass., asked Gen. Joseph Votel, who oversees US operations in Yemen, to clarify whether or not the United States can track whether US refueling support or munitions are used in Saudi coalition airstrikes by the end of the month.

Democrats have also pointed to another Yemen provision in the defense bill, which Trump has also threatened to ignore. The provision calls for the Defense Department to submit a review examining whether US or Saudi coalition forces have violated US or international law in Yemen.

Additionally, Rep. Ted Lieu, D-Calif., on Monday asked the Defense Department’s Office of Inspector General to conduct an independent investigation into possible US legal violations in Yemen on top of the review required in the defense bill.

Nevertheless, Trump’s statement also threatened to withhold information from Congress on the legal review, citing “executive privilege” and his “constitutional authority to withhold information” that could “impair national security, foreign relations, law enforcement or the performance of the president’s constitutional duties.”

Trump’s statement also invoked executive privilege in response to several other reporting requirements in the law. Notably, the law stipulates that the administration must submit strategies on Syria, Iraq and Iran as well as a report on civilian casualties and a legal policy review for advise and assist missions throughout the Middle East and Africa.