Now it’s the Democratic convention that’s promising to get messy.

After piling up millions of votes and wins in 19 states, Bernie Sanders and his supporters are beginning to lay out their expectations for the Democratic National Convention — and they’re expressing deep frustration with what they see as a wall of party resistance.

The most recent flare-up occurred last week, when Sanders publicly released a letter to Democratic National Committee chairwoman Debbie Wasserman Schultz accusing her of stacking the deck against him on the convention’s standing committees. “[W]e 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,” wrote Sanders, several days after a tense phone conversation with the chairwoman.

According to a Sanders official with knowledge of the call, the senator demanded more representation on the committees but Wasserman Schultz would only assure him that he would have representation. A DNC spokesman declined to characterize the conversation and would only confirm that it took place.

For a party that’s anxious to unite all its factions behind likely nominee Hillary Clinton after a long slog of a primary, it was an inauspicious — and worrisome — start.