Progressives in Congress demanded that President Donald Trump justify his legal rationale for strikes in Yemen that caused dozens of deaths, teeing up a potential move to stop future attacks under the 1973 War Powers Act.

Democrats would face long odds invoking congressional war powers to block Trump as long as Republicans control Congress, but a letter sent to the White House on Wednesday and obtained by The Intercept represents a break from the party’s approach to the attacks against Yemen. Until now, Democrats have centered their critiques on the scandal over the administration’s planning of a March 16 strike on the Signal messaging app.

While progressives are still worried about the Trump administration’s slipshod information security, they are also deeply concerned about what is happening in Yemen, says the letter led by Reps. Pramila Jayapal, D-Wash.; Ro Khanna, D-Calif.; and Val Hoyle, D-Ore., who were joined by 30 members.

“While we share concerns about maritime security in the Red Sea,” the representatives wrote, “we call on your Administration to immediately cease unauthorized use of military force and instead seek specific statutory authorization from Congress before involving the U.S. in an unconstitutional conflict in the Middle East, which risks endangering U.S. military personnel in the region and escalating into a regime-change war.”