House Democrats will hold a vote this week to try to block President Trump from escalating the conflict with Iran, using the War Powers Resolution to impose a time limit on his military action.

Speaker Nancy Pelosi announced the move in a letter to fellow Democrats late Sunday, calling the president’s decision to OK the assassination of Iran’s top general last week a “provocative and disproportionate” mistake.

“This action endangered our service members, diplomats and others by risking a serious escalation of tensions with Iran,” Mrs. Pelosi wrote.

Her answer is to trigger the War Powers Resolution, a 1970s-era law that lays out rules for how presidents can deploy the U.S. military in times when Congress has not declared war.

Mrs. Pelosi said the measure the House will vote on will impose a 30-day limit on military action against Iran.

It would have to pass the Senate as well. It then would face a presidential veto and would require a two-thirds vote of both chambers in order to override the veto and constrain Mr. Trump. Even if Congress cannot muster that kind of support, the House could still deliver a symbolic rebuke with its vote.

Mr. Trump officially sent notification to Congress Saturday under the War Powers Resolution that he had ordered a drone missile strike that killed Qassem Soleimani, who was head of Iran’s Islamic Revolutionary Guard Corps.

The actual text of the notification to Congress was marked classified, which effectively hides the president’s justification from public scrutiny. Mrs. Pelosi, who has seen the document, complained about the secrecy and suggested the president’s reasons were sketchy.

The White House has publicly said Soleimani was plotting attacks against Americans and killing him saved lives. The administration says Mr. Trump was on firm legal footing, saying since the drone strike was conducted in Iraq it falls squarely under the 2002 Authorization for the Use of Military Force in Iraq — a measure passed to give then-President George W. Bush permission to oust that country’s dictator, Saddam Hussein.