A bipartisan group of senators wrote to Secretary of State Tony Blinken on Thursday pressing him to redesignate the Houthis as a Foreign Terrorist Organization. Their letter suggests growing frustration on Capitol Hill over the administration’s refusal to take the step after a year of Houthi attacks on U.S. forces, Israel and commercial shipping lanes.

The administration has faced bipartisan pressure from lawmakers to reinstate the Houthis’ FTO designation since as early as January, when the administration imposed a separate Specially Designated Global Terrorist designation which confers less stringent sanctions authorities.

“While we recognize that your administration has listed the Houthis as a Specially Designated Global Terrorist (SDGT), that designation is nowhere near as impactful as an FTO listing. Designating the Houthis as an FTO would impose meaningful costs on the Houthis and degrade their ability to commit acts of terrorism,” the lawmakers wrote.

“Since the misguided revocation of the Houthis’ FTO designation in 2021, the Houthis, backed by the Iranian regime, have only escalated their efforts to destabilize the Middle East,” they added.

The lawmakers noted that the Houthis’ attacks have continued in spite of U.S. and allied military strikes to destroy and disrupt their capabilities, and said that the U.S. needs to “take further action to impose costs on the Houthis” before Americans are killed in the group’s attacks.

The letter argues that the enhanced designation would “enable the United States to better target the group’s assets and financial support and hold the group accountable.”

The letter was organized by Sens. Jacky Rosen (D-NV) and Marco Rubio (R-FL), co-signed by Sens. Bob Casey (D-PA), Rick Scott (R-FL), Catherine Cortez Masto (D-NV) and Marsha Blackburn (R-TN).

Rosen and Casey are both running in competitive re-election races in swing states, and have both previously been outspoken about the need to redesignate the Houthis.

Casey’s support for this letter, describing the initial decision to rescind the FTO designation as “misguided”  is notable given that he he signed and has taken flak on the campaign trail for a May 2021 letter which described the rescinding of the FTO designation as one of several “positive steps” the Biden administration had taken to address the situation in Yemen.

Bipartisan House-passed legislation that seeks to bypass the administration and reimpose the FTO designation has been pending in the Senate for months.