More than 100 lawmakers, including 21 Democrats, have signed a letter that says President Obama would violate the Constitution by striking Syria without first getting authorization from Congress.

A total of 140 lawmakers had signed the letter as of Thursday, highlighting bipartisan interest and growing momentum in ensuring a role for Congress in any decision to use force in Syria. The letter has yet to be sent to the White House.

“Engaging our military in Syria when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers that is clearly delineated in the Constitution,” states the letter, spearheaded by Rep. Scott Rigell Edward (Scott) Scott RigellEx-Rep. Scott Taylor to seek old Virginia seat GOP rushes to embrace Trump GOP lawmaker appears in Gary Johnson ad MORE (R-Va.).

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“I’m grateful and encouraged by the strong, bipartisan support this letter has received,” Rigell said in a statement this evening. “It’s a clear indication that this issue is not personal to the president, but rather represents common ground in Congress and a deep respect for the Constitution.”

The Democrats who have signed the letter so far are Reps. Zoe Lofgren (Calif.), Rush Holt (N.J.), Beto O’Rourke (Texas), Peter DeFazio (Ore.), Kurt Schrader (Ore.), William Enyart (Ill.), Tim Walz (Minn.), Sam Farr Samuel (Sam) Sharon FarrMedical marijuana supporters hopeful about government funding bill Marijuana advocates to give away free joints on Capitol Hill DEA decision against reclassifying marijuana ignores public opinion MORE (Calif.), Bruce Braley Bruce Lowell BraleyThe Memo: Trump attacks on Harris risk backfiring 2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP MORE, (Iowa), Jim McDermott James (Jim) Adelbert McDermottSondland has 'no intention of resigning,' associate says Three women accuse Gordon Sondland of sexual misconduct Portland hotel chain founded by Trump ambassador says boycott is attack on employees MORE (Wash.), Michael Capuano (Mass.), Anna Eshoo (Calif.), Earl Blumenauer Earl BlumenauerAhead of a coronavirus vaccine, Mexico's drug pricing to have far-reaching impacts on Americans Trump threatens to double down on Portland in other major cities Federal agents deployed to Portland did not have training in riot control: NYT MORE (Ore.), Peter Welch Peter Francis WelchShakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Vermont Rep. Peter Welch easily wins primary Vermont has a chance to show how bipartisanship can tackle systemic racism MORE (Vt.), Rick Nolan (Minn.), David Loebsack (Iowa), Jim Matheson James (Jim) David MathesonTrump EPA eases standards for coal ash disposal Utah redistricting reform measure likely to qualify for ballot Trump's budget targets affordable, reliable power MORE (Utah), Collin Peterson (Minn.), Elizabeth Esty (Conn.), Daniel Lipinski, (Ill.) and Dan Maffei (N.Y.).

Current Homeland Security Committee Chairman Mike McCaul (R-Texas) is among the Republicans who has signed onto Rigell’s effort. However, his predecessor, former Homeland Security Committee Chairman Pete King (R-N.Y.), has repeatedly said on TV news programs that Obama does not need authorization from Congress to move forward.

Rigell has called on Obama to bring Congress back to Washington early to consider the authorization of military force against Syria.

“If you deem that military action in Syria is necessary, Congress can reconvene at your convenience,” the letter states.

The congressman also wrote that the president’s decision to authorize the 2011 U.S. military in Libya without Congress’s green light was unconstitutional.

News reports suggest U.S. strikes on Syria could begin as early as Thursday. Congress is not set to return to Washington until Sept. 9.

The administration has reached out to discuss its thinking with lawmakers on several committees, as well as Speaker John Boehner John Andrew BoehnerLongtime House parliamentarian to step down Five things we learned from this year's primaries Bad blood between Pelosi, Meadows complicates coronavirus talks MORE (R-Ohio).

—This story was first posted at 10:25 a.m. on Aug. 28 and last updated at 3:01 p.m. on Aug. 29.