Five military veterans advising Sen. Sinema resign, calling her one of the 'principal obstacles to progress'

(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