WASHINGTON – President Donald Trump's decision to pull U.S. troops from Syria, clearing the way for Turkey's assault on Kurdish allies who helped in the fight against the Islamic State, has created a rare rift with congressional Republicans who have sharply criticized the decision.

For the president, the timing is poor.

Trump's move has helped unite Democrats and Republicans against him at a moment when he needs conservatives in Congress to fend off a fast-moving impeachment inquiry looking into the president's efforts to push Ukraine to investigate former Vice President Joe Biden, one of his top political rivals.

Sen. Lindsey Graham, a South Carolina Republican and a close Trump ally, has been one of the harshest GOP critics of Trump's decision, tweeting last week that abandoning the Kurds would "put every radical Islamist on steroids.”

Minutes after Trump tweeted Monday afternoon that he “would much rather focus on our Southern Border” to protect U.S. security, Graham shot back that the southern border “should be our LAST line of defense against radical Islam."

"Our FIRST line of defense is the U.S. military working with partners in radical Islam’s backyard keeping them over there so they can’t hurt us here at home or hurt our allies like Israel,” Graham wrote.

Other GOP senators, including Marco Rubio of Florida, Cory Gardner of Colorado, and Mitt Romney of Utah, have joined Graham in blasting the president's pullout.

With criticism ramping up from all sides, Trump announced late Monday afternoon that his administration was imposing sanctions on Turkey as his administration called for an "immediate" ceasefire in the fragile region. He said steel tariffs would increase to 50% from 25% and that his administration would stop talks on a proposed $100 billion trade deal with Ankara.

Support from Republicans who control the Senate will be critical for Trump in the coming months. House Speaker Nancy Pelosi, D-Calif., launched the impeachment inquiry last month. The Constitution gives the House the option of bringing articles of impeachment against a president – similar to an indictment in a court proceeding.

If the Democratic-led House decides to impeach Trump, the Senate would have to hold a trial with senators acting as jurors.

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Removal from office would require at least two-thirds of the Senate, or 67 votes.

"This is the worst possible time for Trump to have a major break with Senate Republicans, " said Alex Conant, a GOP strategist and former aide to Rubio. "At the end of the day, he needs those Republican senators to fight for him. And that’s hard to do when they’re fighting with him on Syria."

But another Republican strategist, Ford O'Connell, said he didn't think the Syria issue would make GOP lawmakers any less likely to defend Trump on impeachment.

"They've compartmentalized it largely as a policy difference with Trump," O'Connell said of the Syria pullout. "They knew that he's been saying this and he prides himself as someone who tries to deliver on his promises."

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Republican senators have concluded they can't win in 2020 without Trump on the ballot, O'Connell said. He pointed out that every Senate seat up for election in 2016 was won by the party whose presidential nominee also captured that state.

"They recognize that their ability to hold the Senate after 2020 depends on their political livelihood," O'Connell said.

Still, at a time when polls show voters more open to impeachment, the GOP split over Syria offers a rare point of disagreement that Democrats may try to exploit given how little both parties are working together on big-ticket issues.

On Monday, Graham and Pelosi spoke to each other about Syria.

"Our first order of business was to agree that we must have a bipartisan, bicameral joint resolution to overturn the president’s dangerous decision in Syria immediately," Pelosi said of the phone call.

Graham has drafted legislation that would put sanctions on Turkey in response to its moves in Syria.

Over the past five days, ISIS fighters have escaped prisons and at least 130,000 people have been displaced from their homes as Turkish fighters pushed into northern Syria into towns and villages. All while Kurds, abandoned in battle, turned to Russia and Syrian President Bashar Assad – U.S. foes – for help.

"I thought you were going to defeat ISIS, that is why people voted for you. What changed?" Rep. Adam Kinzinger, R-Ill., who served in the Air Force, wrote on Twitter Monday in a message to Trump. "This is weakness. America is far more honorable than this."

Conant, the former Rubio aide-turned-strategist, warned that the biggest danger Syria poses to Trump isn't impeachment. It's the 2020 election.

Many GOP senators who won in 2016 outperformed Trump in their states and he'll need the enthusiasm of Republican candidates running in battleground states next year to win, he said.

The Associated Press contributed to this article.