President Donald Trump seems to be losing support among fellow Republicans for his withdrawal of U.S. forces from northern Syria.

Trump suffered a major setback when Senate Majority Leader Mitch McConnell, R-Ky., reaffirmed his opposition to the administration's policy of retreating from northern Syria.

In a Washington Post essay published Sunday, McConnell wrote that Trump was reversing a successful long-term U.S. policy of building coalitions in the fight against ISIS terrorists and standing united with allies around the world. "Withdrawing U.S. forces from Syria is a grave strategic mistake," McConnell wrote." It will leave the American people and homeland less safe, embolden our enemies, and weaken important alliances."

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McConnell added that the threat from "radical Islamic terrorism" is "real and cannot be wished away. These fanatics threaten American interests and American lives. If permitted to regroup and establish havens, they will bring terror to our shores....The combination of a U.S. pullback and the escalating Turkish-Kurdish hostilities is creating a strategic nightmare for our country."

McConnell has been one of Trump's most important loyalists in Congress, backing the president on nearly all of his major policy decisions, such as his Supreme Court nominations and cutting taxes. But Trump's Syria withdrawal was too much for the majority leader, whose essay presumably reflected the view of many GOP senators.

Last week, 129 House Republicans supported a resolution condemning the withdrawal. This was a majority of GOP members in the House. They joined majority House Democrats in condemning Trump's move.

Sen. Mitt Romney, R-Utah, has called for Senate hearings into the Syria pullout.