A bipartisan group of more than two dozen House lawmakers plans to send a letter to President Biden on Friday arguing that he must seek authorization from Congress before launching additional strikes against the Houthis in Yemen.

While the signatories comprise a small portion of the House of Representatives, their letter marks some of the strongest opposition to date to Biden’s handling of the widening hostilities in the Middle East. It follows a letter from four senators of both parties earlier this week questioning the strategic and legal rationale for the strikes.

The House letter — led by Rep. Ro Khanna (D-Calif.) and Rep. Warren Davidson (R-Ohio) — is more strongly worded, arguing that the U.S. strikes in Yemen are unauthorized and violate the Constitution. The lawmakers say that the strikes in Yemen — which were planned weeks ahead of time — do not meet the criteria for a “national emergency” that would allow Biden to bypass Congress.

“We ask that your Administration outline for us the legal authority used to conduct these strikes, and we urge your administration to seek authorization from Congress before conducting any more unauthorized strikes in Yemen,” the lawmakers wrote.

Near-daily strikes over the past two weeks on the Houthis, a powerful faction in Yemen’s long-running civil war, have failed to stop the militant group from targeting ships off the Arabian Peninsula. The Houthis have said their campaign against the ships is a way of pressuring Israel in its war in Gaza and standing up for the Palestinians.

Biden last week conceded the U.S. strikes were not entirely effective, but he said the administration would continue them.

“When you say working, are they stopping the Houthis? No,” Biden said in response to a question. “Are they going to continue? Yes.”

The House members signing the letter are mostly members of the Congressional Progressive Caucus, the most left-leaning faction of the Democratic Party, and the Freedom Caucus, the most conservative group in the GOP.

In an interview, Khanna said it was “questionable” whether the first strikes by the administration needed authorization from Congress, but now that the strikes have become routine over a period of two weeks, there is no doubt that approval is required.

“You can’t get us into another Middle East conflict without coming to Congress. Bipartisan majorities on both sides are tired of presidents getting us into Middle East wars without congressional authorization,” Khanna said. “By striking and not coming to Congress, they’re cutting off the debate over what would be effective policy.”

White House officials say they are confident they are in compliance with domestic and international law.

“We are confident, after consultation with the Justice Department and interagency lawyers, that U.S. actions alongside coalition partners against Houthi targets are consistent with international and domestic law,” a White House official said, speaking on the condition of anonymity to discuss internal deliberations. “We have explained that legal basis in various forms, including in public notifications to Congress consistent with the War Powers Resolution and in public letters to the United Nations Security Council.”

The War Powers Resolution of 1973, intended to clarify the constitutional provision giving Congress the power to declare war, requires that the president “in every possible instance” consult with Congress before introducing U.S. military forces into hostilities, and notify Congress within 48 hours regardless. It was passed amid congressional frustration with how American presidents prosecuted the Korean and Vietnam wars, and tension between the White House and Congress over the use of the military has simmered ever since.

A second White House official noted that the Houthi attacks in the Red Sea have gone beyond commercial vessels. The official said U.S. government ships have been attacked by the militant group and that the administration is authorized to act in self-defense.

The Senate letter, sent to the White House on Tuesday, warned Biden that he will need congressional approval to carry the United States deeper into an escalating Middle East war.

“Unless there is a need to repel a sudden attack, the Constitution requires that the United States not engage in military action absent a favorable vote of Congress,” Sens. Tim Kaine (D-Va.), Chris Murphy (D-Conn.), Todd Young (R-Ind.) and Mike Lee (R-Utah) wrote in the letter.

They said that while the defense of U.S. personnel and military assets — and perhaps even commercial vessels — is “within the boundaries of … presidential power … most vessels transiting through the Red Sea are not U.S. ships, which raises questions about the extent to which these authorities can be exercised.”