Washington — The House will vote Thursday on a resolution to prohibit President Trump from engaging in hostilities in Iran under the 1973 War Powers Resolution, as Democrats attempt to reassert Congress’ constitutional authority to declare war amid a tense standoff with the Islamic Republic.

Speaker Nancy Pelosi unveiled the resolution on Wednesday, saying it would be sent to the House floor for a vote on Thursday. The resolution is sponsored by Congresswoman Elissa Slotkin, a freshman Democrat and former analyst for the Central Intelligence Agency and the Department of Defense.

The president authorized a strike on Iranian military leader Qassem Soleimani last week without consulting or notifying Congress, incurring outrage from Democratic lawmakers as well as some Republicans.

Thursday’s measure directs the president to “terminate the use of United States Armed Forces” against Iran without congressional authorization, except when necessary to “defend against an imminent armed attack.”

The resolution will almost certainly pass in the House, where Democrats have the majority. Restrictions on Mr. Trump’s authority face dimmer prospects in the Senate, but cracks in the president’s support among Republican senators began to emerge late Wednesday afternoon on Capitol Hill.

Republican Senator Mike Lee of Utah fumed to reporters after a briefing by top administration officials about the strike that killed Soleimani and the intelligence underlying the legal justification for taking him out. Lee said the officials’ presentation was so inadequate that it convinced him to support a similar war powers resolution backed by Democratic Senator Tim Kaine. Lee called the briefing “probably the worst briefing at least on a military issue I’ve seen,” saying it was “insulting.”

“They had to leave after 75 minutes while in the process of telling us we need to be good little boys and girls and not debate this in public,” Lee said. Republican Senator Rand Paul also said the inadequacy of the briefing had convinced him to support Kaine’s war powers measure.

The resolution in the House would direct the president to end hostilities with Iran under a section of the War Powers Resolution of 1973. That resolution, intended to constrain President Richard Nixon’s powers in the final throes of the Vietnam War, says that any forces engaged in hostilities outside the U.S. “shall be removed by the President if the Congress so directs by concurrent resolution.”

A concurrent resolution would not require President Trump’s signature to take effect. But legal questions about Congress’ authority to direct the executive branch via concurrent resolution remain unresolved. In 1983, the Supreme Court struck down a similar provision dealing with concurrent resolutions in another law, according to an analysis by the Congressional Research Service.

Pelosi also said Wednesday that the House may soon vote on legislation to repeal the 2002 Authorization for Use of Military Force, as well as additional legislation to prevent the president from spending funds on hostilities with Iran without explicit congressional authorization.

Earlier, Mr. Trump appeared to support de-escalation of tensions between the U.S. and Iran, after Iran launched ballistic rocket strikes targeting military bases housing U.S. forces in Iraq on Tuesday in response to Soleimani’s death.

“I’m pleased to inform you, the American people should be extremely grateful and happy. No Americans were harmed in last night’s attack by the Iranian regime,” the president said in an address to the nation from the White House on Wednesday. “We suffered no casualties, all of our soldiers are safe and only minimal damage was sustained at our military bases.”

Stefan Becket and Rebecca Kaplan contributed to this report

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