WASHINGTON — Four key national security Democrats in Congress voiced strong reservations Friday to President Donald Trump potentially launching a strike against Syria, and they blasted Trump for appearing to waffle in recent days.

Three Senate Democrats, in a letter, pressed Trump for his rationale and legal justification for a potential strike before he takes action. In a separate statement, House Armed Services Committee’s top Democrat, Rep. Adam Smith, flatly opposed military action against Syrian President Bashar Assad, questioning whether it would improve life for Syrians.

“Iraq provides a lesson in the limitations of the military’s ability to unilaterally resolve massive global crises,” Smith said. “At this point, we need a clear strategy from the President. What we don’t need are schizophrenic tweets that leave our allies confused.”

House Minority Leader Nancy Pelosi, D-Calif., told reporters on Thursday that Trump needs a new military force authorization, or AUMF, before he can launch a strike in Syria. But House Speaker Paul Ryan, R-Wis., has said Trump already has the authority he needs.

“He has the authority under the existing AUMF,” Ryan said Thursday. “What I would hate to do is have an AUMF that ties the hands of our military behind their backs. That makes it much more difficult to respond, to keep us safe, because they have the authority to do that now.”

Sens. Jack Reed, D-R.I., ranking member of the Senate Armed Services Committee; Dick Durbin, D-Ill., Senate minority whip and vice chairman of the defense appropriations subcommittee, and Bob Menendez, D-N.J., ranking member of the Senate Foreign Relations Committee, sent Trump the letter asking that he provide his legal justification.

“This issue is of critical importance and the American people should be fully informed about your rationale for deploying American military power and the objectives of any U.S. military action in Syria,” the senators wrote. “As previous Commanders-in-Chief have done in similar situations, we believe you should present a clear public articulation of these matters to the American people at the earliest appropriate time.”

The Assad regime’s “continuing abuse and murder of the Syrian people is abhorrent,” and the world is “rightly horrified” by its use of chemical weapons, the senators wrote. Yet, using the military against another nation, “is a momentous decision that poses serious risks to the lives of U.S. military personnel involved and the possibility of escalation into a broader conflict.”

The external debate comes alongside the administration’s internal debate. Top military officials, including Defense Secretary James Mattis, reportedly warned Trump in a meeting Thursday that he risks escalating U.S. involvement in Syria if he goes forward with the type of aggressive bombing campaign he has pressed for over the past week.

Trump has pushed military leaders to develop plans for a sustained assault on Syrian regime targets in response to last weekend’s chemical attack, CNN reported Friday, citing U.S. and western officials briefed on the conversation. Yet Mattis and other members of Trump’s national security team cautioned Trump during the meeting that such a strategy could pull the US into direct conflict with Russia and Iran.

Last week, images from a chemical attack in the Syrian town of Douma emerged, indicating Syrian government involvement. That prompted Trump to hint in a tweet at imminent military action against the Syrian regime. U.S. missiles “will be coming, nice and new and ‘smart,’” he said, also taunting Russian leaders for claims they would defend their Syrian allies if such an attack took place.

But, just hours before Mattis testified on Capitol Hill a day later, Trump appeared to partially reverse course on those threats, tweeting that he “never said when an attack on Syria would take place. Could be very soon or not so soon at all!”

Those comments drew a host of questions for Mattis from members of the House Armed Services Committee, who questioned the legality of such action, the potential impact on U.S. forces in the region and the value of publicly announcing military strikes before they occur.

Mattis dismissed lawmaker concerns, asserting that the existing AUMF cover such moves. He said the U.S. role in Syria is to defeat the Islamic State and the threat of chemical weapons use in a region where U.S. troops are already operating represents a clear threat to America.