WASHINGTON — President Barack Obama’s request for congressional backing to train and arm rebels battling Islamic State militants in Syria is halfway home after its easy approval by the GOP-controlled House sent the issue to the Senate, where leaders in both parties say approval is ensured.

Obama won support from staunch Republicans who typically are reflexively against him and lost the votes from some of his most loyal Democratic allies in the 273-156 House tally. Republicans backed Obama by a more than 2-1 margin; Democrats backed him as well, but to a lesser degree.

Top leaders of both parties stood with the president despite reservations that his strategy of arming moderate rebel groups could backfire or won’t be enough to blunt the advance of Islamic State forces. Obama has pledged airstrikes as well but is adamant that he won’t send U.S. combat troops to battle the Islamic extremists.

“We must pursue a comprehensive and sustained counterterrorism strategy, and a bipartisan coalition in the House voted to support a critical component of that strategy,” Obama said after the vote.

The Senate was to vote today on the measure, which was added to a must-pass, stopgap spending bill to keep government agencies operating into December.

The measure is the last major business on Capitol Hill before lawmakers depart this week to return to their districts and states to campaign for re-election.

The new authority is part of $500 million that Obama requested in May to train and equip rebels. The cost, to be covered by leftover war funding from this year, generated virtually no discussion among lawmakers, who focused instead on the possible consequences of a new military mission not long after a war-exhausted nation largely pulled out of Afghanistan and Iraq.

Testifying before a Senate committee Wednesday, Secretary of State John Kerry said the forces seeking to create an Islamic caliphate “must be defeated. Period. End of story.”

On that there was agreement. The question now is whether Obama’s plan will work. GOP hawks called the president’s approach too little, too late, even as many of them supported it as a first step in a broader campaign against Islamic State extremists, who have taken large swaths of Iraq and Syria and shocked the world by beheading two American journalists and a British aid worker.

“Committing insufficient force in any conflict is self-defeating, and airstrikes alone cannot win a war,” said Rep. Tom McClintock, R-Calif., who voted “nay.”

Despite their doubts, top House Republicans saw little choice but to back the president.

“I am not convinced this train-and-equip effort will change the balance of power on the ground anytime soon, and I believe this approach comes with great risks,” Majority Leader Kevin McCarthy, R-Calif., said. But he also said “voting against this request would send a terrible message” about America’s determination and willingness to stand with its allies.

Democrats proved to be a harder sell, backing Obama 114-85.

“We simply don’t know if somewhere down the line it will turn our guns back against us,” said Rep. Loretta Sanchez, D-Calif.