A bipartisan group of lawmakers sent President Trump a letter Wednesday urging him to withdraw U.S. troops from Syria within the next six months.

Trump has already committed to pulling back U.S. forces from the region, but Sen. Rand Paul, R-Ky., Rep. Ro Khanna, D-Calif., and others say it needs to happen quickly.

“We write in bipartisan support of your announcement of the start of a “deliberate withdrawal” of U.S. military forces in Syria, and we welcome the completion of this process within the next six months.” Paul wrote to Trump in a letter that was also signed by Rep. Alexandria Ocasio-Cortez, D-N.Y., a star progressive in the House freshman class. “It is long past time to rein in the use of force that goes beyond congressional authorization, and we look forward to pursuing this longstanding bipartisan objective with your Administration.”

Rep. Ilhan Omar, D-Minn., another freshman who was recently rebuked for anti-Semitic tweets, also signed the letter, along with Sen. Mike Lee, R-Utah, Reps. Justin Amash, R-Mich., Andy Biggs, R-Ariz., Jeff Duncan, R-S.C., Paul Gosar, R-Ariz., Thomas Massie, R-Ky., and Bill Posey, R-Fla.

[Opinion: Shock unity: Rand Paul, Ocasio-Cortez, Ilhan Omar, Ro Khanna, Mike Lee, herald Trump’s Syria withdrawal]

The group belongs to a faction of Congress that believes the U.S. involvement in Syria is not authorized by Congress although many other lawmakers say it is sanctioned by congressional war authorization provided in 2001.

Trump announced in December that the U.S. would end its involvement in the longstanding Syrian conflict aimed at eliminating Islamic State terrorists. Trump declared victory against ISIS in the region but some lawmakers feared the absence of U.S. military in Syria will lead to encroachment by Iran and Russia.

But Paul, other Republicans and many liberal Democrats believe the United States should never have sent troops into Syria without a new authorization vote from Congress. Lawmakers have dodged such a vote over the past two decades.

“The president cannot pursue a foreign policy agenda without the advice and consent, let alone the support, of the Congress,” Khanna said.