CNN  — 

Five military veterans on Sen. Kyrsten Sinema’s advisory board resigned from their roles this week, slamming the Arizona Democrat as one of the “principal obstacles to progress.”

In a letter to Sinema, the veterans expressed frustration with her refusal to change the Senate filibuster to protect voting rights, failure to support prescription drug negotiations, her opposition to parts of the Democrats’ sweeping budget reconciliation package that make up President Joe Biden’s agenda and criticized her for not voting on the January 6 commission.

“You have become one of the principal obstacles to progress, answering to big donors rather than your own people,” the veterans wrote. “We shouldn’t have to buy representation from you, and your failure to stand by your people and see their urgent needs is alarming.”

“We do not know who has your ear, but it clearly isn’t us or your constituents,” they added.

The criticism from the five veterans, whose resignations were effective Wednesday, adds to the growing backlash within the Democratic Party and from Arizona activists against Sinema for not backing elements of Biden’s Build Back Better plan.

Their letter will be featured in a new ad from the progressive veterans group Common Defense, which will air on TV in Phoenix and Tucson.

The New York Times was first to report on the veterans’ resignations and the ad.

Members of Sinema’s veterans advisory council are voluntary positions and not paid members of her staff.

The group of five military veterans wrote they felt “as though we are merely given performative titles and used as window dressing for your own image—not as resources to provide counsel on what’s best for veterans.”

“We no longer feel you are aligned with our values, and we cannot in good faith continue to serve on your council,” they said.

Sinema told CNN in a statement that she would “always remain grateful for these individuals’ service to our nation,” and had valued their input to her work.

“While it is unfortunate that apparent disagreement on separate policy issues has led to this decision,” she said. “I thank them for their service and will continue working every day to deliver for Arizona’s veterans who have sacrificed so much to keep us safe and secure.”

This story has been updated with additional details.

CNN’s Kevin Liptak contributed to this report.