A bipartisan group of senators sent a letter to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Wednesday asking him to reconsider his decision to begin withdrawing troops from Syria.

In the letter, Sens. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.), Jeanne Shaheen Cynthia (Jeanne) Jeanne ShaheenSenate Democrats introduce bill to sanction Russians over Taliban bounties Trump-backed candidate wins NH GOP Senate primary to take on Shaheen Democratic senator urges Trump to respond to Russian aggression MORE (D-N.H.), Joni Ernst Joni Kay ErnstThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Romney backs pre-election Supreme Court vote, paving way for McConnell, Trump MORE (R-Iowa), Angus King Angus KingGovernment watchdog recommends creation of White House cyber director position Democrats step up hardball tactics as Supreme Court fight heats up Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' MORE (I-Maine), Tom Cotton Tom Bryant CottonGOP brushes back charges of hypocrisy in Supreme Court fight Trump uses bin Laden raid to attack Biden Battle lines drawn on precedent in Supreme Court fight MORE (R-Ark.) and Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (R-Fla.), expressed concern that withdrawing U.S. troops from Syria would "renew and embolden" ISIS's efforts in the region.

"We write today to express our concern regarding your Administration’s consideration to withdraw U.S. forces from Syria," the senators wrote. "We believe that such action at this time is a premature and costly mistake that not only threatens the safety and security of the United States, but also emboldens ISIS, Bashar al Assad, Iran, and Russia."

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“If you decide to follow through with your decision to pull our troops out of Syria, any remnants of ISIS in Syria will surely renew and embolden their efforts in the region," they continued.

"However, ISIS is not the only threat. The brutal dictatorship of Bashar al Assad continues to weigh heavily upon the Syrian people, and we fear that a withdrawal of our troops may embolden Bashar al Assad to take further actions to solidify his power,” they added.

The senators also said they believed removing American presence from Syria would give a boost to "two other adversaries," Iran and Russia.

"As you are aware, both Iran and Russia have used the Syrian conflict as a stage to magnify their influence in the region," the senators wrote. "Any sign of weakness perceived by Iran or Russia will only result in their increased presence in the region and a decrease in the trust of our partners and allies."

The senators urged the president not to "repeat the same mistakes" made by administrations before his.

"Your administration must not repeat the same mistakes that previous administrations have made and concede to these bad actors,” they wrote.

Trump declared victory against ISIS in Syria on Wednesday morning in a tweet after The Wall Street Journal, followed by The Washington Post, reported that he was preparing an immediate withdrawal of U.S. troops from Syria.

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,” the president tweeted.

The White House confirmed in a statement Wednesday that the administration has “started returning United States troops home” but emphasized that coalition efforts in Syria would continue. It also doubled down on Trump's claim that “the United States has defeated the territorial caliphate.”

"The United States and our allies stand ready to re-engage at all levels to defend American interests whenever necessary, and we will continue to work together to deny radical Islamist terrorists territory, funding, support, and any means of infiltrating our borders,” White House press secretary Sarah Huckabee Sanders said in a statement.

Senate Republicans, in a rare move, lashed out at Trump over the decision, with several lawmakers saying Congress received no warning of Trump’s announcement.

“It’s a terrible decision,” Rubio said earlier Wednesday. “I hope it can be entirely or at least partially reversed, or we’re going to pay a big price for it in the years to come.”

“I don’t know what they’ve done, but this is chaos,” Sen. Lindsey Graham (R-S.C.), a staunch Trump ally and Armed Services Committee member, told reporters earlier that day.

— Updated 10:30 p.m.