A group of Democratic senators, including two 2020 presidential candidates, demanded that the king of Saudi Arabia release political prisoners as Congress debates whether to end U.S. military aid for the kingdom’s war in Yemen.

In a letter addressed to King Salman sent Tuesday, the senators wrote that human rights abuses in Saudi Arabia threaten the delicate U.S.-Saudi relationship, which has severely soured after the killing of Washington Post contributing columnist Jamal Khashoggi last fall.

Lawmakers on Capitol Hill have been critical of the Trump White House’s tepid response after Khashoggi’s death. President Trump has refused to link Saudi Crown Prince Mohammed bin Salman to the Oct. 2 slaying, despite the CIA’s conclusion that Mohammed ordered the killing.

Last week, the Senate voted for the second time to end U.S. support for the Saudi-led war in Yemen. The House is expected to take up the same resolution, but it’s unlikely Trump will sign it, given his support for Saudi Arabia and reluctance to upset that alliance.

The letter, signed by Sens. Richard J. Durbin (Ill.), Patrick J. Leahy (Vt.), Dianne Feinstein (Calif.), Kirsten Gillibrand (N.Y.), Richard Blumenthal (Conn.), Elizabeth Warren (Mass.), Ron Wyden (Ore.), Edward J. Markey (Mass.) and Jeanne Shaheen (N.H.), asks specifically for the release of 12 political prisoners, including Walid Fitaihi, an American citizen who reportedly has been tortured throughout his captivity.

“The brutality of the Khashoggi killing and Saudi-led intervention in Yemen, as well as the continued detention of political prisoners, threaten not only Saudi Arabia’s regional role but also the future of the U.S.-Saudi relationship,” the senators wrote in the letter to King Salman. “We will continue to closely watch Saudi action — or inaction — on human rights as Congress considers measures related to the Middle East. Our shared interests must be underscored by support for basic values and freedoms, as anything else will not be sustainable.”

A Senate Democratic aide said that there were efforts to get Republicans to sign onto the letter but that none had chosen to do so.

Last week’s vote on ending U.S. military aid to Saudi Arabia had the support of seven Republicans. Instead, more Republicans have backed proposals to impose additional sanctions against Saudi Arabia and block arms sales, although the Trump administration has not been supportive of those crackdowns, either.