WASHINGTON — President Donald Trump is facing harsh criticism from fellow Republicans over his decision to declare victory over the Islamic State and withdraw U.S. forces from Syria.

Sen. Lindsey Graham, R-S.C., a close Trump ally, said it would be an "Obama-like mistake" to remove American troops. Sen. Marco Rubio, R-Fla., called it a "grave error" and Rep. Adam Kinzinger, R-Ill., said Trump's declaration that ISIS has been defeated is "simply not true."

"An American withdrawal at this time would be a big win for ISIS, Iran, Bashar al Assad of Syria, and Russia," Graham said in a statement. "I fear it will lead to devastating consequences for our nation, the region, and throughout the world."

Earlier in the day, two senior defense officials and a third person familiar with the plan told NBC News that the U.S. is preparing to withdraw a significant number of the roughly 2,000 troops that remain in Syria.

Trump has been telegraphing the move for most of the past year at campaign rallies and in other venues.

"We're knocking the hell out of ISIS," he said in March. "We'll be coming out of Syria, like, very soon. Let the other people take care of it now."

On Wednesday, he tweeted news of the withdrawal decision: "We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency."

Trump allies have long believed that the defeat of ISIS makes for a compelling campaign narrative for a president who promised to reduce American interventionism as he seeks re-election in 2020. Before last month's midterms, there was a more immediate risk that a withdrawal followed by a flare-up in violence in the region could have negative political consequences for Republican candidates.

Sen. Rand Paul, R-Ky., who is a member of the Senate Foreign Relations Committee, said he is supportive of Trump’s choice to withdraw troops from Syria, and what makes us less safe is having troops involved in “one war theater after another."

"I think it takes courage to do this because there’s a million naysayers on both sides of the aisle that always want to remain and that's why we’re everywhere," Paul said. "We have troops in so many countries, we’re fighting everywhere because no one knows how to declare victory. So I’m very supportive of the president’s declaration. I'm very supportive of bringing the troops home."

Still, several GOP lawmakers issued blistering rebukes of Trump's decision on the policy merits Wednesday. The basic argument is that the removal of U.S. forces will create a power vacuum that could empower Iran, Syria, Russia, terrorist groups or some combination thereof.

Sen. Ben Sasse, R-Neb., a member of the Armed Services committee, called the move a "weak decision."

"Eight days ago the Administration called a hypothetical pullout ‘reckless.’ Today, we're leaving. The President's generals have no idea where this weak decision came from: They believe the high-fiving winners today are Iran, ISIS, and Hezbollah," Sasse said. "The losers are Israel, humanitarian victims, and U.S. intelligence gathering. A lot of American allies will be slaughtered if this retreat is implemented.”

Sen. Cory Gardner, R-Colo., who is up for re-election in 2020 in a state that has been trending toward Democrats, pressed Trump to reconsider.

"We've made significant progress in our fight against ISIS but the fight isn't over, & a US withdrawal will embolden bad actors," he said in a tweet.

The US is the only global power able to stop Russia, Iran, & their terrorist proxies in Syria from total control of a region vital to national security. We've made significant progress in our fight against ISIS but the fight isn't over, & a US withdrawal will embolden bad actors. — Cory Gardner (@SenCoryGardner) December 19, 2018

Rubio said there are three reasons why it's a "colossal mistake" to withdraw from Syria: ISIS has been converted into an insurgency and will be a more powerful one without the U.S. presence, Syria will fall more under the control of Russia and Iran without U.S. forces and the U.S. will be more readily seen as an "unreliable ally" by the rest of the world.

"I just think it's a bad decision that eventually will lead to greater risk for the United States," Rubio said. "I'm all for peace. I would love to live in a world in which U.S. deployment of service men and women abroad were not necessary. That's not the world we live in."