WASHINGTON (AP) — One day after President Barack Obama sent Congress legislation backing the use of military force against Islamic State militants, he's still searching for his first outright supporter for the measure.

Republicans and Democrats, House and Senate, senior lawmakers and newcomers to Congress, lawmakers across the congressional spectrum all found parts of the proposal to oppose, or else said nothing to tip their hand.

Rep. Ed Royce, R-Calif., chairman of the House Foreign Affairs Committee, said Thursday he was pleased that Obama had formally asked Congress to authorize use of military force against the Islamic State.

But Royce said Obama still "needs to make the case to the American people" and Congress, adding, "This won't be easy."

Over the past year, the Foreign Affairs Committee has pressed the Obama administration to intensify and accelerate its response to the threat posed by the Islamic State, Royce said at a hearing Thursday as the panel reviewed Obama's request.

"Some pieces are being put together, but too slowly," Royce said, noting that about 85 percent of all airstrikes against the Islamic State group are from U.S. fighter jets.

"This air campaign isn't pummeling the enemy as it should," Royce said. "Congressional authority is of no value if the president isn't willing to act decisively."

In Obama's own party, Sen. Patrick Leahy of Vermont, the senior Democrat in the Senate, referred to two wars in the past decade and a half and said any legislation must avoid "repeating the missteps of the past." He added it must stay clear of an "open-ended authorization that becomes legal justification for future actions against unknown enemies, in unknown places, at unknown times."

Rep. Ted Lieu of California, a first-term Democrat, said he didn't think Obama had yet made the case that the Islamic State terrorist group "represents a direct, grave threat to the United States."

Republicans control a majority in both the House and Senate, and made clear they will insist on changes. They focused, in part, on Obama's call to rule out "enduring offensive combat operations," while leaving the door open to a more limited role.

House Speaker John Boehner expressed doubt Wednesday that it would "give our military commanders the flexibility and authorities they need to succeed and protect our people."

Sen. John McCain, R-Ariz., a frequent critic of Obama's foreign policy, was unsparing. He said Obama had omitted air support for U.S.-trained rebels battling Syrian President Bashar Assad, adding, "that's immoral."

The House and Senate each intend to hold hearings on the proposal, although administration officials are not expected to be asked to testify until Congress returns from a one-week vacation that begins on Friday.

Instead, the House Foreign Affairs Committee arranged to hear testimony during the day from non-government witnesses.

Obama was resolute as he made the case for legislation in remarks at the White House.

"Make no mistake. This is a difficult mission," he said, calling for action against a group that he said threatens U.S. security.

He said it will take time to dislodge the terrorists, especially from urban areas, "but our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose."

Under Obama's proposal, the use of military force against Islamic State fighters would be authorized for three years, unbounded by national borders. The fight could be extended to any "closely related successor entity" to the Islamic State organization that has overrun parts of Iraq and Syria, imposed a stern form of Sharia law and killed several hostages it has taken, Americans among them.

While asking lawmakers to bar long-term, large-scale ground combat operations like those in Afghanistan and Iraq, Obama said he wants the flexibility for ground combat operations "in other more limited circumstances." Those include rescue missions, intelligence collection and the use of special operations forces in possible military action against Islamic State leaders.

While he proposed legislation to terminate in three years, Obama said, "It is not a timetable. It is not announcing that the mission is completed at any given period. What it is saying is that Congress should revisit the issue at the beginning of the next president's term."

The president's action set Congress on a path to its first war powers vote since 2002, when President George W. Bush won approval for the U.S invasion in Iraq.

It also likely injected a new issue into the 2016 race for the White House, just as the 2002 vote became a campaign point of contention between then-Sen. Hillary Rodham Clinton, who voted in favor of the legislation, and Obama, who criticized her for it.

Clinton declined to comment Wednesday on Obama's proposal, but Republicans eyeing the White House were not as reticent.

Florida Sen. Marco Rubio faulted the president for not asking for more. "What we need to be authorizing the president to do is to destroy them and to defeat them, and allow the commander in chief — both the one we have now and the one who will follow — to put in place the tactics, the military tactics, necessary," he said in remarks on the Senate floor.

Sen. Lindsey Graham, R-S.C., said Obama's plan was "fatally flawed" and should be altered to permit shielding U.S.-trained rebels who are battling Syrian government forces.