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 RigellGOP rushes to embrace Trump GOP lawmaker appears in Gary Johnson ad Some in GOP say Trump has gone too far 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 FarrSam 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 BraleyOPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer MORE, (Iowa), Jim McDermottJim McDermottLobbying World Dem lawmaker: Israel's accusations start of 'war on the American government' Dem to Trump on House floor: ‘Stop tweeting’ MORE (Wash.), Michael Capuano (Mass.), Anna Eshoo (Calif.), Earl BlumenauerEarl BlumenauerDemocrat: Pelosi ‘ceded the moral high ground’ on sexual harassment Clyburn on disparity in responses to sexual allegations: ‘Who elected them?’ Third House Dem calls for Conyers to resign MORE (Ore.), Peter WelchPeter Francis WelchTrump talks tough but little action seen on drug prices Frustrated with Trump, Dems introduce drug pricing bill Lawmakers try again on miners’ pension bill MORE (Vt.), Rick Nolan (Minn.), David Loebsack (Iowa), Jim MathesonJim MathesonTrump's budget targets affordable, reliable power Work begins on T infrastructure plan New president, new Congress, new opportunity 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 BoehnerJohn Feehery: A political forest fire Trump's pick for Federal Reserve chief is right choice at right time The two-party system is dying — let’s put it out of its misery 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.