In a Friday letter, Sen. Bernie Sanders (I-VT) accused Democratic National Committee Chair Debbie Wasserman Schultz of stacking committees at the July national convention with supporters of Hillary Clinton.

Sanders lamented to Wasserman Schultz that the DNC only nominated three of the more than 40 people he recommended to sit on the standing committees, and the DNC did not appoint any Sanders picks to the key Rules Committee.

“I believe the composition of the standing committees must reflect the relative support that has been received by both campaigns,” Sanders wrote in the letter. “That was why I was so disappointed to learn that of the over forty people our campaign submitted at your request you chose to select only three of my recommendations for the three standing committees. Moreover, you did not assign even one of the people submitted by our campaign to the very important Rules Committee of the Democratic National Convention.”

Sanders wrote that the lack of representation for his supporters at the convention suggests that the Democratic Party “is not open to the millions of new people that our campaign has brought into the political process.”

He also wrote that he was disappointed in the process for choosing members of the committee that drafts the Democratic Party platform at the convention, and he suggested that he and Clinton each recommend seven members of the committee and jointly select the committee chair.

“In our conversation, you told me with respect to the platform Drafting Committee that you would consider allowing each campaign to submit ten names from which you would choose four from each and then you would add an additional seven,” he wrote. “While having four members on the Drafting Committee is an improvement, it does not address the fact that up to this point Bernie 2016 has secured 45% of the pledged delegates awarded. Frankly, we believe that percentage will go up in the coming weeks and, of course, we hope it will end up being a majority.”

Sanders threatened to have his delegates attempt to make changes to the party platform and the convention rules at the July convention if the DNC does not address his grievances.

“It is my hope we can quickly resolve this in a fair way. If the process is set up to produce an unfair, one-sided result, we are prepared to mobilize our delegates to force as many votes as necessary to amend the platform and rules on the floor of the convention,” he wrote.

The DNC responded to Sanders’ letter in a Friday statement, according to Politico.

“Because the Party’s platform is a statement of our values, the DNC is committed to an open, inclusive and representative process,” the statement read. “Both of our campaigns will be represented on the Drafting Committee, and just as we did in 2008 and 2012, the public will have opportunities to participate.”