(Bastien Inzaurralde,Jenny Starrs/The Washington Post)

Congress offered a mix of reactions on Friday to President Trump’s military action in Syria, with many lawmakers welcoming the airstrikes, some lambasting Trump for failing to seek congressional approval, and nearly all calling for more information and collaboration from the White House.

Friday’s operation, the first direct U.S. assault on the Syrian government in six years of bloody civil war, surprised members as they prepared to leave Washington for a two-week recess and reignited a debate over the legality of U.S. military actionthat lacks explicit approval from Congress.

Divisions quickly emerged on this question between Republican leaders and rank-and-file conservatives, with Senate Majority Leader Mitch McConnell (R-Ky.) and Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) arguing no formal approval by Congress — known as an Authorization for Use of Military Force, or AUMF — was necessary for the initial round of airstrikes.

“It was a measured response to a chemical weapons attack,” McConnell said in an interview. “ I don’t believe [the president] needed authority from us to do what he did.”

Prominent conservatives, however, pointedly criticized Trump’s decision to act without buy-in from lawmakers.

(Jayne Orenstein/The Washington Post)

“The President needs Congressional authorization for military action as required by the Constitution, and I call on him to come to Congress for a proper debate,” Sen. Rand Paul (R-Ky.) said in a statement, arguing that the intervention would do “nothing to make us safer.”

Rep. Justin Amash (R-Mich.), one of the most vocal libertarians in the House, said it was “unwise to base military actions on emotional responses.”

“We have to think about all the consequences,” he said Friday. “It’s critical under our system of government that these types of actions have congressional approval, because they are acts of war, and what begins as a set of strikes on one night can quickly escalate into a much broader conflict.”

Vice President Pence is expected to provide an explanation for the airstrikes to Congress within a few days, lawmakers said.

The mix of reactions from Capitol Hill underscored the delicate situation facing Trump after the first major military operation of his presidency. Once again, Trump is receiving blowback from his right flank while Democrats seek ways to capi­tal­ize on the Republican infighting and maintain pressure on an unpopular president.

Democrats on Friday criticized Trump, who has long opposed military intervention against Syrian President Bashar al-Assad, for suddenly changing his mind after a deadly chemical attack on civilians this week.

(Erin Patrick O'Connor/The Washington Post)

“Three days ago he was effectively giving Syria back to Assad,” Sen. Chris Murphy (D-Conn.), a member of the Foreign Relations Committee, said of Trump. “Two days later, admittedly after a serious chemical weapons attack, we’re launching unauthorized military strikes. . . . There is no strategy on Syria.”

Murphy also called for Congress to debate an AUMF that would govern U.S. actions against the Islamic State and any further intervention against Assad.

“If Mitch McConnell really is an institutionalist, then he shouldn’t be so willing to cede war-making authority to the executive,” Murphy said.

The strongest support for the airstrikes came Friday from Republican leaders, who praised Trump’s decisiveness and said he was well within his powers.

“The chemical weapons attack committed by the Assad regime was a flagrant violation of international standards, and preventing a deepening of the humanitarian crisis and instability in Syria is clearly in the United States’ national interest,” said AshLee Strong, a spokeswoman for House Speaker Paul D. Ryan (R-Wis.).

“As such, last night’s response was fully within the president’s authority,” she said.

The question of what new AUMF , if any, is necessary for U.S. military operations in the region has been debated for years.

The Obama and Trump administrations have carried out strikes against the Islamic State under AUMFs adopted in the early 2000s for the wars in Afghanistan and Iraq, though many lawmakers are skeptical about that argument. Neither of the current authorizations address potential hostilities against the Syrian government.

Rank-and-file Republicans and Democrats were eager Friday to assert lawmakers’ constitutional ability to authorize force, though few are in agreement about what a new proposal should cover.

In the House, Rep. Adam B. Schiff (D-Calif.) promised to reintroduce an AUMF proposal against the Islamic State and al-Qaeda, while Sens. Jeff Flake (R-Ariz.) and Tim Kaine (D-Va.) are renegotiating a proposal to address such extremists within and outside of Syria. Sen. John McCain (R-Ariz.) is working on an even broader proposal that he said would be more of a “policy” than an authorization “on [a] specific issue.”

Asked if the AUMF would be global in breadth, McCain said, “It applies to everything.”

Some Republicans were less convinced a new AUMF is necessary.

McConnell said he would be interested in taking a look at a proposal “if the president feels like he needs it.” Sen. Lindsey O. Graham (R-S.C.) said that the Constitution “does not require the president to get congressional approval to use military force.” And Corker said that an AUMF might be necessary only if Trump decides to launch several more airstrikes against the Syrian regime.

Conservatives like Paul argue that Congress should be able to consider whether to authorize force.

“I’m sure there will be ongoing discussions with the administration and Congress concerning last night’s attack, the legal justification, and any proposed military action going forward,” Sen. Ted Cruz (R-Tex.) said. “I’ll wait to assess the administration’s argument.”

“If the United States is to increase our use of military force in Syria, we should follow the Constitution and seek the proper authorization from Congress,” Sen. Mike Lee (R-Utah) said.

Senators huddled Friday afternoon with Chairman of the Joint Chiefs of Staff Joseph F. Dunford Jr. for a briefing on Syria and what might come next. Murphy, who was in the room, said Dunford “essentially punted” on the question of whether the administration needs congressional authorization for strikes against Syrian military targets.

Paul, who said he has not been contacted by anyone at the White House about the airstrikes, called the briefing “a day late and a dollar short.”

“It’s unconstitutional,” he said of the airstrikes. “It might work, but it’s not constitutional to do it the way we have done it.”

The two-week spring recess will prevent Congress from immediate action, and Republican leaders showed no sign they planned to call lawmakers back to Washington because of the airstrikes.

Nonetheless, a bipartisan group of senators introduced legislation Friday designed to increase pressure on Assad by tracking war crimes in Syria and supporting “investigations and other credible transitional justice efforts,” such as an international tribunal, to hold the regime accountable.

Some lawmakers also called for further action to punish Russia, Assad’s chief ally.

McConnell on Friday suggested that he would look favorably on a proposal to step up sanctions against Russia, Iran and others who support the regime in Syria. Such a measure passed in the House last year, but was never taken up in the Senate.

Mike DeBonis, Sean Sullivan, Ed O’Keefe and Dave Weigel contributed to this report.