In separate statements, the leading Republicans in Congress, Speaker Mike Johnson and Senator John Thune of South Dakota, the majority leader, commended the military operation, calling it a necessary check on Iran's ambitions of developing a nuclear weapon. Both men had been briefed on the military action before the strike was carried out, according to three people familiar with the matter who were not authorized to discuss it publicly.
Mr. Johnson and Mr. Thune both argued that the airstrikes were necessary after Iran had rejected diplomatic overtures to curb its nuclear program.
"The regime in Iran, which has committed itself to bringing 'death to America' and wiping Israel off the map, has rejected all diplomatic pathways to peace," Mr. Thune said.
Mr. Johnson argued that the military action was consistent with Mr. Trump's muscular foreign policy.
"President Trump has been consistent and clear that a nuclear-armed Iran will not be tolerated," he said. "That posture has now been enforced with strength, precision and clarity."
But top Democrats, who were given only perfunctory notice of the strikes before they occurred, harshly criticized the move.
"President Trump misled the country about his intentions, failed to seek congressional authorization for the use of military force and risks American entanglement in a potentially disastrous war in the Middle East," Representative Hakeem Jeffries, Democrat of New York and the minority leader, said in a statement. He said the president "shoulders complete and total responsibility for any adverse consequences that flow from his unilateral military action."
Senator Chuck Schumer, Democrat of New York and the minority leader, demanded "clear answers" from Mr. Trump on the operation and called for an immediate vote on legislation that would require explicit authorization from Congress for the use of military force.
"The danger of wider, longer, and more devastating war has now dramatically increased," he said.
Representative Jim Himes, the ranking Democrat on the Intelligence Committee, condemned the operation as unconstitutional and warned that it could drag the United States into a larger conflict.
"Donald Trump's decision to launch direct military action against Iran without congressional approval is a clear violation of the Constitution, which grants the power to declare war explicitly to Congress," he said in a statement. "It is impossible to know at this stage whether this operation accomplished its objectives. We also don't know if this will lead to further escalation in the region and attacks against our forces, events that could easily pull us even deeper into a war in the Middle East."
While Senator Tom Cotton, Republican of Arkansas and the chairman of the Intelligence Committee, called Mr. Trump's move "the right call," the top Democrat on the panel, Senator Mark Warner of Virginia, said he had taken steps that could drag the United States into a war "without consulting Congress, without a clear strategy, without regard to the consistent conclusions of the intelligence community, and without explaining to the American people what's at stake."
Leading national security Democrats on Capitol Hill were not informed of the strikes until after Mr. Trump had posted about them on social media, according to three people familiar with the matter who would discuss it only on the condition of anonymity.
And one high-profile Democrat, Representative Alexandria Ocasio-Cortez, called the operation grounds for impeachment.
"He has impulsively risked launching a war that may ensnare us for generations. It is absolutely and clearly grounds for impeachment," Ms. Ocasio-Cortez of New York said in a post on social media.
Democrats widely condemned the surprise attack as unconstitutional. But Ms. Ocasio-Cortez was the first on Saturday to say it was grounds for Trump's removal, breaking with party leaders who have avoided talk of impeachment since the president returned to the White House after two failed attempts to remove him during his first term. The Constitution gives Congress the authority to declare war, but in modern times, presidents of both parties have unilaterally carried out attacks on other countries without congressional authorization. It has been decades since Congress voted on whether to authorize military force, and efforts to claw back the legislative branch's war powers have repeatedly stalled.
Most of the praise immediately following the operation in Iran came from Republicans, many of whom argued that the bombings would not lead to a ground deployment of American forces in the region.
"To those concerned about U.S. involvement -- this isn't a 'forever war' in fact, it's ending one," Senator Markwayne Mullin, Republican of Oklahoma, said on social media.
Senator Roger Wicker, the Republican chairman of the Armed Services Committee, called Mr. Trump's decision to strike in Iran "deliberate" and "correct."
"We now have very serious choices ahead to provide security for our citizens and our allies and stability for the Middle East," Mr. Wicker said in a statement.
Senator John Fetterman of Pennsylvania, whose unqualified support for Israel has put him at odds with other members of his party, was one of the few Democrats to offer an immediate statement of support. He wrote on social media that the military action "was the correct move."
"Iran is the world's leading sponsor of terrorism and cannot have nuclear capabilities," Mr. Fetterman added. "I'm grateful for and salute the finest military in the world."
Other lawmakers, many of them Democrats who had already expressed concerns that the Trump administration was considering sidestepping Congress's constitutional power to declare war, immediately criticized the strikes on the nuclear sites.
Mr. Trump, "did not come to Congress to explain his reasons for bombing a sovereign nation and to seek authorization for these strikes," Representative Diana DeGette, Democrat of Colorado, said in a statement. "These reckless actions are going to put the lives of American service members and American citizens at risk."
Representative Thomas Massie, Republican of Kentucky, who earlier this week introduced a bipartisan resolution that would require congressional approval before U.S. troops could engage in offensive attacks against Iran, wrote on social media that the attack was "not Constitutional."