Sen. Rand Paul (R-Ky.) told Foreign Policy that he would “work with a bipartisan coalition to explore forcing a vote on blocking this sale. Saudi Arabia is an unreliable ally with a poor human rights record. We should not rush to sell them advanced arms and promote an arms race in the Middle East.”

Congressional opposition to the arms sale came as the Saudi-led, U.S.-backed military coalition broke an unsteady five-month ceasefire in Yemen last week and resumed bombing in the capital city of Sana’a—prompting immediate reports of civilian deaths. On Saturday, Doctors Without Borders/Médecins Sans Frontières (MSF) reported that an airstrike on a school in northern Yemen killed 10 children and wounded 28 others.

Paul and Sen. Chris Murphy (D-Conn.), both of whom sit on the Foreign Relations Committee, are outspoken critics of the coalition.

“If you talk to Yemeni Americans, they will tell you in Yemen this isn’t a Saudi bombing campaign, it’s a U.S. bombing campaign,” Murphy said in June. “Every single civilian death inside Yemen is attributable to the United States.”

Congress has 30 days after arms sales are announced to block or modify the deal, but actual intervention is rare.

A number of human rights organizations have opposed the deal. Oxfam and CODEPINK, among others, launched a petition to “[f]orce a public debate on U.S. participation in the Saudi war in Yemen by advocating for blocking the planned transfer of U.S. tanks and armored vehicles to Saudi Arabia,” which as of Monday had collected 9,500 signatures.

And this specific arms deal is especially important, according to foreign policy experts. As Robert Naiman explains, “In this particular case, it’s plausible that if we can block the Saudi arms sale, or even come close and have a robust public food fight about it, we can help end the catastrophic Yemen war.”

The effort is also supported by many lawmakers in the House of Representatives. Over the weekend, Rep. Ted Lieu (D-Calif.), warned that “the Saudi military’s operational conduct in Yemen and the killing of civilians with U.S.-made weapons have harmed our national security interests, and I will continue to oppose any arms sale that contributes to its operations in that arena.”

“This approved sale deserves to be scrutinized by Congress rather than rubber-stamped during the summer recess,” he said.