On the eve of a House vote Thursday, Defense Secretary Mark Esper urged Congress not to debate limits to President Donald Trump’s power to strike Iran because doing so might embolden Tehran and hurt U.S. troops, multiple sources tell ABC News.

The suggestion by Esper, in a classified briefing for lawmakers on Wednesday, enraged some members, including Utah Republican Sen. Mike Lee, who swiftly marched to the television cameras following the 75-minute briefing to declare it “insulting.” Lee said the briefing felt like being told to be “good little boys and girls and run along and not debate this in public.”

“I find that absolutely insane,” he said.

Jonathan Hoffman, chief Pentagon spokesman, said Esper was responding to a “direct question” on the matter when he expressed concern “about unintentional messages to the force” if Congress attempted to repeal an existing authorization for military force in Iraq “at the same time that American troops are in harm’s way.”

“In follow up questioning, the Secretary made it clear that he was not questioning the appropriateness of Congressional debate on military force, but rather he stated a concern about how such a debate would be interpreted by the force,” Hoffman said in a statement provided to ABC News.

That Esper – a Cabinet member and former senior staffer on Capitol Hill – suggested lawmakers would be wise to stand down on the matter of providing a constitutional check to the executive branch is likely to become a flash point in Congress and unify Democrats with libertarian-leaning conservatives like Sens. Lee and Rand Paul of Kentucky.

Under the Constitution, Congress has the power to declare war and can decide whether to pay for military operations. Several lawmakers have expressed a desire to try to find a way to curb Trump’s war powers, and the House was expected to vote Thursday on a resolution.

The president, though, is the nation’s commander in chief charged with protecting the nation. Even with the 1973 War Powers Act, Democrats struggled during the Bush administration to try to curb operations in Iraq and bring troops home. One concern was that voting to end spending for military operations could hurt American service members.

The debate on presidential war powers resurfaced this week after Tehran fired more than 20 ballistic missiles into Iraq, targeting two military facilities that house American troops. The Iranian government took credit for the strikes and said they were in retaliation for a recent U.S. drone strike that killed one of its top generals, who the U.S. accuses of plotting an attack on American service members and diplomats.

Upon being briefed on the matter, Republicans mostly emerged with praise for Trump’s handling of the crisis.

Sen. James Risch, R-Idaho, called it “one of the best briefings I’ve ever had” and said the intelligence supporting the U.S. drone strike was “crystal clear.”

But Democrats bristled at the brevity of the meeting on such a weighty topic and said they still weren’t convinced Trump needed to kill the Iranian military general to protect U.S. assets. Sen. Chuck Schumer, D-N.Y., said that only 15 of the 97 senators attending were able to ask questions before it ended abruptly.

“The questions began to get tough, and they walked out,” Schumer said.

Sen. Jeff Merkley, an Oregon Democrat, said in an interview with MSNBC that the briefers suggested a congressional debate on military operations against Iran would result in “disrespecting our troops.”

Secretary of State Mike Pompeo, CIA Director Gina Haspel and Chairman of the Joint Chiefs of Staff Mark Milley joined Esper in briefing the lawmakers.

“The idea that Congress is not here to have any role in the issues of war and peace is exactly contrary to the Constitution – a Constitution that this president doesn’t respect,” Merkley said.

House Speaker Nancy Pelosi said Wednesday that the upcoming resolution would call for an end to military hostilities against Iran in 30 days, absent any additional congressional action.

Pelosi said she also wants the House to take up legislation that would repeal the 2002 authorization of military force in Iraq. Trump’s national security adviser, Robert O’Brien, cited that resolution as legal justification for the drone strike that killed Gen.Qassem Soleimani. The 2002 resolution authorized the president to use force to “defend the national security of the United States against the continuing threat posed by Iraq.”

Pelosi says Trump’s powers must be kept in check.

“America and the world cannot afford war,” she said.

ABC News' Elizabeth McLaughlin contributed to this report.