President Barack Obama is flanked by Vice President Joe Biden (L) and Secretary of State John Kerry (R) as he delivers a statement on legislation sent to Congress.

WASHINGTON — President Obama on Wednesday sent Congress his long-awaited formal request to authorize military force against “Islamic State,” meeting swift resistance from Republicans as well as his fellow Democrats wary of another war in the Middle East.

Republicans, who control Congress and say Obama’s foreign policy is too passive, want stronger measures against the militants than outlined in the plan, which bars any large-scale invasion by U.S. ground troops and covers the next three years.

Obama acknowledged that the military campaign is difficult and will remain so. “But our coalition is on the offensive. ISIL is on the defensive and ISIL is going to lose,” he said in a televised statement from the White House.

With many of Obama’s fellow Democrats insisting the plan is too broad because it includes no blanket ban on ground troops, it could be difficult for the authorization to pass, even though six months have passed since the campaign began.

Obama consulted with Republicans and Democrats in writing the resolution, and said he would continue to do so. He said the time frame was intended to let Congress revisit the issue when the next president takes office in 2017.

The proposal says “Islamic State” “has committed despicable acts of violence and mass execution.” Its militants have killed thousands of civilians while seizing territory in Iraq and Syria in an attempt to establish a hub of jihadism in the heart of the Arab world.

They have also generated international outrage by beheading western aid workers and journalists and burning to death a Jordanian pilot.

Obama sent his request to Congress a day after his administration confirmed the death of Kayla Mueller, a 26-year-old aid worker who was the last known American hostage held by the group.

Both the Senate and House of Representatives must approve Obama’s plan. Lawmakers said they would begin hearings quickly as Republicans made clear they thought the plan fell short.

The Republican Speaker of the House of Representatives, John Boehner, told reporters he was sure the plan would change as it moved though Congress. “I’m not sure the strategy that has been outlined will accomplish the mission the president says he wants to accomplish,” he added.

Obama has defended his authority to lead an international coalition against Islamic State since Aug. 8 when U.S. fighter jets began attacks in Iraq. The formal request eased criticism of Obama’s failure to seek the backing of Congress, where some accused him of breaching his constitutional authority.