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 RigellScott RigellSpanberger's GOP challenger raises over .8 million in third quarter Ex-Rep. Scott Taylor to seek old Virginia seat GOP rushes to embrace Trump 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 FarrSamuel (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 BraleyBruce 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 McDermottJames (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 BlumenauerEarl BlumenauerLawmakers spend more on personal security in wake of insurrection On The Money: Schumer pressured from all sides on spending strategy | GOP hammers HUD chief over sluggish rental aid | Democrat proposes taxes on commercial space flights Hillicon Valley: Biden to appoint Big Tech critic to DOJ antitrust role | House passes host of bills to strengthen cybersecurity in wake of attacks | Bezos returns from flight to space MORE (Ore.), Peter WelchPeter Francis WelchShakespeare gets a congressional hearing in this year's 'Will on the Hill' Democrats debate shape of new Jan. 6 probe On the Money: Tech giants face rising pressure from shareholder activists | House Democrats urge IRS to reverse Trump-era rule reducing donor disclosure | Sen. Warren, Jamie Dimon spar over overdraft fees at Senate hearing MORE (Vt.), Rick Nolan (Minn.), David Loebsack (Iowa), Jim MathesonJames (Jim) David MathesonMcAdams concedes to Owens in competitive Utah district Trump EPA eases standards for coal ash disposal Utah redistricting reform measure likely to qualify for ballot 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 BoehnerJohn Andrew BoehnerFreedom Caucus presses McCarthy to force vote to oust Pelosi Stripping opportunity from DC's children Here's what Congress is reading at the beach this summer 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.