WASHINGTON — President Barack Obama’s possible military intervention in Syria is already running into fierce opposition among some members of Congress, with a growing chorus of Republican and Democratic lawmakers demanding he seek congressional authorization for any strikes against the Assad regime.

In the House, Republican Rep. Scott Rigell of Virginia is asking colleagues to sign a letter to Obama urging him to reconvene Congress and seek approval for any military action. And in the Senate, even some who support punishing the Syrian government for launching alleged chemical weapons attacks are joining the call for the president to first gain Congress’ approval.

“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,” Rigell’s letter argues.