Bernie Sanders’ supporters are mad as hell. And they’re not going to take it anymore.

They think the Democratic primary process is rigged against him. Or that he’s not getting a fair amount of positive media coverage. Some are convinced the superdelegate system is designed to keep him from winning the nomination.

In short, they have lots of grievances — grievances that could complicate attempts to unify the party at the end of an increasingly contentious primary season.

“I think that there has always been this presumption that Secretary Clinton is the prohibitive favorite, [that] she has this thing locked up so the Sanders campaign has been this fool’s errand,” said Neil Sroka, the communications director for Democracy for America, which has endorsed Sanders. “And so I guess it’s kind of like, to quote Rodney Dangerfield, you never get any respect.”

The superdelegate system utilized by the Democratic Party ranks high among the frustrations of Sanders’ supporters. The notion that unelected delegates could act counter to the popular vote in states the Vermont senator has won sticks in their craw, resulting in deep resentment at the grass-roots level.

“I think it’s safe to say that activists on the ground are confused and dismayed by the superdelegate system,” Bill Lipton, New York state director for the Working Families Party, which backs Sanders, said. “On the surface it seems to fail the test of basic democracy.”

Severin Beliveau, a former Maine Democratic Party chairman and former state lawmaker, said after the Vermont senator’s 64 percent to 35 percent win over Clinton in Maine he unsuccessfully urged Rep. Chellie Pingree, one of the state’s three Clinton superdelegates, to vote with the state.

“I said, ‘Maine overwhelmingly supported Sanders. I assume when you’re going to have to cast your vote that you would recognize this and not vote against your constituents,’” Beliveau said.

Beliveau, who supports Sanders, said there’s a strong sense of frustration over how Clinton has built a large superdelegate count despite Sanders’ recent spate of wins.

“I think what you’re going to find is ultimately the sense of frustration and almost betrayal by the Democratic establishment,” said Beliveau. “As you know, all over the country Clinton enjoys the support of all the party leaders, but in most of those states that support her it hasn’t produced the results that she had thought because we all believe Sanders is speaking out on issues that affect the majority of Americans, and that he has become the voice of the frustrated Democrats, and independents, for that matter, and some Republicans.”

Stephanie Ohigashi, chairwoman of the Hawaii Democratic Party, said she has been inundated with pressure from Sanders supporters demanding she support Sanders after he won the state’s caucuses with roughly 70 percent of the vote to Clinton’s 30 percent.

“They’ve been sending me emails,” Ohigashi said. “It’s from the mainland. It’s from other states. They’ve been sending emails, they’ve been calling my house.”

Spencer Thayer, a Chicago activist, went so far as to create a website with the URL superdelegatehitlist.com. The site lists all the names of superdelegates, as well as addresses and phone numbers for many of them.

In an email, Thayer argued that the “superdelegate system is outdated and anti-Democratic.”

Asked about complaints that some Clinton superdelegates said they were feeling harassed, Thayer said, “These superdelegates shouldn’t be surprised that voters are holding their vote to account. If they don’t want to be contacted, they can easily pass the torch to a representative who is prepared to deal with the public.”

Sanders supporters say their frustration over the superdelegate system is compounded by what they consider to be insufficient attention focused on the Vermont senator. Pro-Sanders reddit forums are devoted to the idea, with featured threads on the news media ignoring Sanders or giving him unfair coverage.

A recent Pew Research survey found that 58 percent of Sanders backers say the Vermont senator has received too little coverage.

Maine state Rep. Diane Russell, a Sanders supporter who is hoping to change party rules at the state Democratic convention in May so that superdelegate support would align more closely with the popular vote, argued that there’s always a negative spin on Sanders coverage.

“When he does get coverage, it’s ‘great, he’s won another state, but he can’t win,’” Russell said of the general tone of media coverage. “Well, he keeps winning!”

Recent Western contests have also drawn the ire of Sanders supporters, from the delegate allocation in Wyoming — where Sanders won 56 percent of the caucus vote but only broke even in pledged delegates with Clinton — to long lines in Arizona, where Sanders partisans believe he was disadvantaged in the primary by hours-long waits at some polling locations, which they viewed as an attempt at voter suppression.

“There is a very strong sense of frustration among the Bernie activists that the system is not fair, from young people not being able to vote because of rules about their college IDs to not being able to register on the same day or change their registration. I mean there were hundreds and hundreds of people in line here in Arizona all the way up to 12 o’clock at night who got told they would not get to vote even though they waited three or four hours. That kind of frustration is very rampant,” said Dan O’Neal, an organizer for the Arizona chapter of the Progressive Democrats of America.

Democrats say the prospect of Donald Trump or Ted Cruz as president is galvanizing enough on the left that they’re not worried that the party will be divided after the nomination is settled in July. The concern, however, is that it might dampen Democratic turnout if Clinton is the nominee.

“I think it could hurt enthusiasm, that is Hillary’s biggest issue,” Democratic strategist Bill Hyers said. “But they will vote for her at the end of the day against Trump or Cruz.”

“Most people I’m sure will vote for Hillary if she’s the nominee because no one wants a Trump or a Cruz,” added O’Neal. “There will be a percentage of people, I don’t know [what percentage], that will stay away out of total frustration and may be politically unsophisticated a little bit. And then there’s going to be hopefully the organized forces that will say, ‘the genie’s out of the bottle. Bernie Sanders is not going away.’“