Before the U.S. military attacked Saddam Hussein’s forces in 1991, Congress spent months considering its approval of the war. Twelve years later, another attack on Iraq came after three weeks of debate and intensive negotiations.

But this week, the House and Senate each took only a few hours to sign off on President Obama’s plan to once again deploy the military in the Middle East, approving it as part of a broader budget bill before bounding out of town for a two-month recess and hitting the midterm campaign trail.

Some on Capitol Hill were stunned by the seeming congressional disinterest on a matter of such importance. Now, lawmakers in both parties are asking an uncomfortable question: Has Congress forfeited its rightful role in the most solemn of all national decisions?

Some in leadership have said a more expansive debate over U.S. actions against the Islamic State militant group might come after the Nov. 4 elections. Yet a few dozen lawmakers are starting to speak up, complaining that such vague promises are not enough and that Congress has a duty to set the parameters of the nation’s new war posture.

The sentiment was expressed Friday in a stern letter from a dozen lawmakers — six Democrats and six Republicans — who wrote to House leaders in both parties, “The time has come to take up and debate an authorization regarding U.S. military operations in Iraq.”

The letter said, “We believe such a debate and vote is required, will enhance our national security, and the ability of Congress and the Executive to carryout U.S. foreign and defense policies abroad.” Signatories included Reps. Tom Cole (R-Okla.) and Jim McGovern (D-Mass.), who are close to their party’s respective leaders.

Many lawmakers believe such a brief debate on something that felt akin to approving a small war, and then leaving for nearly two months, sent a bad signal for an institution already held in low regard by the voting public. Others contend that Obama should start the process by formally sending a request to the Capitol, as did George H.W. Bush and George W. Bush in formally starting the earlier Iraq war debates.

“The way it should work is, [White House officials] come up with a piece of legislation, they send it over, we work on it and amend it, then we reach agreement and then we pass it,” Sen. John McCain (R-Ariz.) said.

The relative lack of debate this week belied the tensions in both parties over the extent to which the United States should remain entangled in overseas conflicts.

Both chambers approved the measure with wide bipartisan majorities, but there were visible signs of discord. Eighty-five House Democrats — more than 40 percent of the caucus — bucked the president, demonstrating the antiwar left has a new rallying point. In the Senate, several rising stars who are considered potential presidential candidates — including Sens. Rand Paul (R-Ky.) and Elizabeth Warren (D-Mass.) — rejected the proposal.

The issue was left in a muddled state. Sens. Robert ­Menendez (D-N.J.) and Bob Corker (R-Tenn.), leaders of the Senate Foreign Relations Committee, both support drafting a resolution that would broadly define the field of combat in Iraq and Syria, but aides said there have been no serious bipartisan talks. Many rank-and-file lawmakers believe that, with the initial request they approved this week technically expiring Dec. 11, a full-fledged debate is unavoidable during the lame-duck session.

Leaders in both parties, however, have provided little clarity on timing. And, although they don’t acknowledge it publicly, neither party wants the fall political debate to turn on military issues.

In a statement after Thursday night’s Senate vote, Majority Leader Harry M. Reid (D-Nev.) said only that he would give the Foreign Relations Committee process “time to unfold” and that he expected a “robust debate to continue on this issue in the months to come.” The Democrats’ leader in the House, Nancy Pelosi (D-Calif.), said Obama’s current plan requires no additional action by Congress, suggesting that there is some undefined “threshold” that would require a war powers debate.

In legislative and diplomatic parlance, Congress rarely declares “war.” Over history, just five formal declarations of war have been made: in the two world wars of the 20th century, the War of 1812, the 1846 war against Mexico and the 1898 war against Spain.

Instead, Congress approves resolutions approving military force overseas with some parameters concerning whom can be attacked and where the attacks can occur. Insiders call this an AUMF: “authorization for use of military force.”

Through such authorizations, Congress has played a central role in debating a number of U.S. military campaigns in the Middle East over the past two decades.

In 1990 and early 1991, after Hussein’s forces overran Kuwait, deep divisions in Congress prompted several months of hearings and informal debate as the first Bush administration amassed hundreds of thousands of troops near the Kuwaiti border.

Some Democrats sued to try to prevent Bush from using military force, citing the War Powers Act, but in early January 1991 he sought congressional approval. The House held three full days of debate, an unusually long time. On narrow votes, the White House won.

A decade later, in the immediate aftermath of the Sept. 11, 2001, terrorist attacks, Congress granted authorization for the use of force against those responsible for the attacks.

In the fall of 2002, just before a midterm election, the debate over invading Iraq began on Capitol Hill.

The formal request arrived on Sept. 19 of that year, and immediately senior members of Congress began working on it. A measure was approved by mid-October with large bipartisan backing.

Some lawmakers fear that this week’s action will be akin to the Gulf of Tonkin Resolution, which was approved in 1964 after President Lyndon B. Johnson said North Vietnamese forces had launched two unprovoked attacks on U.S. ships off Vietnam. The resolution stated that the United States was ready to do all it could militarily in the region but stopped short of openly approving military action. Presidents in both parties went on to cite that resolution — approved unanimously in the House — for an escalating war in Vietnam.

Critics from both parties note that Obama’s legal and military advisers have cited the 2001 authorization regarding the Sept. 11 attacks to justify his authority to target Islamic State forces in Iraq and Syria.

“It’s no longer applicable in any way. If you read it, it says ‘those responsible for the attacks of Sept. 11’, so it’s clearly outmoded and needs to be updated,” McCain said.

There was no update this week, however, as the House held six hours of debate on Wednesday, and the Senate held five hours on Thursday. The Senate debate did not fill up the allotted time, so at one point a senator devoted time to praising the Baltimore Orioles for their successful baseball season.

Some warned that Congress was too late to the issue.

“As we say in Montana, the horse is out of the barn, the cows are out to pasture,” Sen. Jon Tester (D-Mont.) said.

Ed O’Keefe, Wesley Lowery and Sebastian Payne contributed to this report.