President Trump unveiled a new strategy for the U.S. war in Afghanistan on Aug. 21. (Victoria Walker/The Washington Post)

President Trump’s decision to announce expanded U.S. military operations in Afghanistan but disclose few details is set to spark fresh congressional debate about the future of the United States’ longest war and whether it is time for lawmakers to approve a new use-of-force law.

Senior Republicans voiced support for Trump’s decision to endorse a Pentagon plan to boost troop levels and said it will be reviewed during public congressional hearings when lawmakers reconvene next month. But Democrats and some Republicans blasted Trump for not disclosing more information and said they will redouble attempts to pass the first use-of-force resolution since the 2001 act that authorized military action against terrorist groups in the wake of the Sept. 11 attacks.

“The majority of us weren’t in Congress in 2001,” said Sen. Tim Kaine (D-Va.), a member of the Foreign Relations Committee who is leading a bipartisan push to approve a new Authorization for Use of Military Force, which some members of both parties think is needed before Trump can deploy more troops. Kaine called on the Senate to “stop dodging its responsibility and finally pass an updated AUMF.”

Sen. Rand Paul (R-Ky.), a longtime critic of expanded military operations, said it is “a terrible idea to send any more troops” into Afghanistan. The 2016 presidential candidate said in a statement that he will keep pushing his plan to repeal the 2001 use-of-force agreement, along with a 2002 resolution that allowed military operations in Iraq, as part of this year’s must-pass defense policy bill.

Sen. Tim Kaine (D-Va.), center, with Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), right, on Aug. 3 at the Capitol. (J. Scott Applewhite/AP)

Trump’s decision to adopt a conditions-based approach to the war without a specific timetable angered Democrats, who suggested the new plan could leave U.S. troops in Afghanistan indefinitely. “He is declaring an open-ended commitment of American lives with no accountability to the American people,” House Minority Leader Nancy Pelosi (D-Calif.) said Monday night.

Although Trump did not specify in his speech how many more troops will be sent to Afghanistan, congressional officials said senior administration officials told them Monday it will be about 4,000, adding to the 8,500 U.S. service members now in the region.

Sen. Jack Reed (D-R.I.), top Democrat on the Armed Services Committee, said in a statement that Trump’s speech “was short on the details our troops and the American people deserve.”

Sen. Kirsten Gillibrand (D-N.Y.), a vocal Trump critic, said the president was “lacking in details, lacking in substance and lacking in a vision of what success in Afghanistan looks like.”

Sen. Tammy Duckworth (D-Ill.), who flew Army helicopter missions during the Iraq War, said the presidential address “was filled with bluster but devoid of details and raises far more questions than it answered.”

And Sen. Chris Murphy (D-Conn.) tweeted, “The only new strategy announced tonight was that strategy will no longer be announced.”

Administration officials including, from right, Secretary of State Rex Tillerson, Treasury Secretary Steven Mnuchin and Attorney General Jeff Sessions watch Trump’s speech Monday from the front row at Fort Myer in Virginia. (Carolyn Kaster/AP)

As a presidential candidate, Trump called it “counterproductive” for the United States to announce troop withdrawal dates. Moving forward, “we will not talk about numbers of troops or our plans for further military activities,” Trump said Monday night. “Conditions on the ground, not arbitrary timetables, will guide our strategy from now on. America’s enemies must never know our plans or believe they can wait us out. I will not say when we are going to attack, but attack we will.”

House Speaker Paul D. Ryan (R-Wis.) credited Trump with embracing a new “doctrine” of “principled realism,” saying on CNN during a nationally televised town hall event that President Barack Obama committed a “strategic mistake” by setting a timetable for troop withdrawals. “We shouldn’t telegraph our timetable when we’re leaving, so that we can actually make it conditions-based, which is, what is the purpose of being there?” Ryan said. “The purpose of being there is to make sure that we don’t have another 9/11, that the Taliban doesn’t give al-Qaeda safe haven to plan and get money and come and have a terrorist attack against us.”

Senate Armed Services Committee Chairman John McCain (R-Ariz.) said that Trump’s new war plan will face congressional scrutiny when lawmakers return to Washington and that “this strategy is long overdue.” But McCain added in a statement that Trump “is now moving us well beyond the prior administration’s failed strategy of merely postponing defeat.”

Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) defended Trump’s new conditions-based approach, saying it “should lead to better diplomatic outcomes, and ensures engagement with regional partners, especially Pakistan and India, giving us a better opportunity for success.”

House Armed Services Committee Chairman Mac Thornberry (R-Tex.) said Trump’s new plan is a “reasonable way ahead that begins with being honest about the requirements needed to win, and the challenges in the region.” In his statement, he called on Congress to “provide timely and adequate funding for this vital mission.”

But how and when lawmakers will debate the new Afghanistan strategy, and how to pay for it, are still not known.

In July, the House passed a $790 billion spending bill that would increase military funding. But it has virtually no chance of becoming law, because it is unlikely to survive in the more closely divided Senate. The bill would blow past a defense spending cap enacted under the 2011 Budget Control Act by $72 billion.

Early versions of the spending bill included a repeal of the 2001 authorization for military action against terrorist groups.

The repeal had earned bipartisan support from House appropriators, but GOP leaders later used procedural moves to strip out the repeal before final approval.

Rep. Barbara Lee (D-Calif.), who wrote the repeal plan, said Monday night that Congress needs to pass a new military force law “before we commit to another surge that will keep our troops in Afghanistan for years to come and cost billions more in spending.”

The Senate, meanwhile, has yet to schedule debate on a spending plan or the National Defense Authorization Act, the bill that sets military policy and was approved by the House in July. Senators had expected to debate and pass the $696 billion measure before the August recess, allowing McCain to lead floor debate on the measure before returning to Arizona to begin treatment for an aggressive form of brain cancer. But Paul objected to beginning debate on the bill, because he had not yet received assurances that his plan to repeal the AUMFs would earn an up-or-down vote.

Congress faces a daunting to-do list when it returns after Labor Day, including a need to raise the federal debt limit and pass a spending plan to keep the federal government open beyond the end of the fiscal year on Sept. 30, plus the reauthorization of a host of federal entities, including the Federal Aviation Administration and the National Flood Insurance Program.

That means Senate debate on the defense bill probably will be pushed to October at the earliest.

Elise Viebeck contributed to this report.