WASHINGTON — Republican and Democratic leaders in Congress will face some resistance in a vote to authorize President Barack Obama’s war against Islamic State militants despite international outrage over video of militants beheading their captives and burning one alive.

War authorizations are among the most difficult issues confronting members of Congress. Several Democrats will be reluctant to approve new war powers unless there is a clear deadline or some way to pay for the military operation. Some Republicans, strong foes of the president, will object to giving Obama the authority.

Obama is poised in coming days to ask Congress for new authority to use U.S. military force against IS, the White House said Thursday. But the top House Republican warned it won’t be easy to pass the measure. Speaker John Boehner, R-Ohio, said it will be up to the president to rally support from lawmakers and the public.

“His actions are going to be an important part of trying for us to get the votes to actually pass an authorization,” Boehner said Thursday. “This is not going to be an easy lift.”

In the U.S. battle against IS, Obama has been relying on congressional authorizations that President George W. Bush used to justify military action after the Sept. 11, 2001, attacks. Critics say the White House’s use of post-9/11 congressional authorizations is a legal stretch, at best.

Obama has insisted that he had the legal authority to send U.S. troops to train and assist Iraqi security forces, and to launch airstrikes since September against targets in Iraq and Syria.

Now, the administration wants to get a new so-called Authorization for the Use of Military Force, or AUMF, with bipartisan support from Congress.

“The president believes it sends a very powerful signal to the American people, to our allies, and even to our enemies, that the United States of America is united behind this strategy to degrade and ultimately destroy ISIL,” White House spokesman Josh Earnest said, using an acronym for the Islamic State group.