Three hundred readers who responded to a Guardian callout expressed serious misgivings about how the Republican and Democratic parties select nominees

An angry Donald Trump supporter in Colorado set fire to his Republican party registration card. The Democratic superdelegates supporting Hillary Clinton are fielding an influx of calls and emails from frustrated Bernie Sanders supporters putting pressure on them to switch candidates.

The rise of anti-establishment candidates like Sanders and Trump has thrown into sharp relief the mess of rules and processes deliberately designed to keep these candidates from clinching the nomination. For these voters, and the many more across the political spectrum just waking up to the ground rules of the highest-stakes game in the country, the primary races can seem horribly unfair.

“Why should the states, the party organizations and the voters go through all the hype, expense and time to have a primary when the party poobahs make the ultimate decision,” said Don Grafues, one of more than 300 readers who responded to a Guardian callout on voter dissatisfaction with the primary process.

In the callout, readers expressed serious misgivings about the way the parties select their nominees, describing the modern primary process as “rigged”, “undemocratic” and a “charade”. A preponderance of respondents who said they felt “cheated” are supporters of Trump or Sanders, the two candidates with broad appeal among voters who feel disenfranchised and excluded from inside-the-Beltway politics.

“Now that we have true grassroots, anti-establishment, populist candidates on both sides [Sanders and Trump], the political machine is in full survival mode to maintain the status quo,” wrote Chris Ritz, a Sanders supporter. “Party leaders sense a real and direct threat from both campaigns … and both major parties are going to use every tool they have to block what they feel is a threat to their very existence.”

Voter anxiety is exacerbated among Republicans who could in effect have their votes nullified if the candidates manage to hold off Trump and force a contested convention. Trump has skillfully played into this mounting sense of futility.

“The system is rigged,” the candidate said on Fox News, claiming that results of last week’s Colorado convention proved the process was designed to shut out insurgent candidates. “I see it now, 100%. And not just on our side, but I think it is worse on the Republican side.”

Last month, Trump warned that supporters could “riot” if he doesn’t secure the nomination at the party’s convention this summer.

These kinds of “shenanigans”, as Trump has called such efforts to obstruct him from winning the nomination, are exactly why Wendy Kranmer, of Turin, New York, is pessimistic about politics.

“The reason I never voted is because I thought of corruption,” Kranmer said, speaking before a Trump rally in Rome. “I thought maybe I was wrong or unfair, but it’s [the electoral process this year] kind of making me think there’s a lot of corruption and your voice doesn’t count.”

Yet she plans to vote, at age 52, for the first time ever, for Trump in the New York primary next week in hopes that it will count.

Meanwhile, Sanders’ rabid fanbase is confounded by the fact that after seven straight victories he has barely dented Clinton’s more than 200-delegate lead. Just this weekend, Sanders won Wyoming by a double-digit margin, yet each candidate walked away with seven delegates.

Sanders supporters are especially upset by the role of superdelegates, a constellation of elected officials and other party elites who can vote for whichever candidate they prefer at the convention – and most are backing Clinton.

“It’s insanely hypocritical for the party that protests voter ID laws to embrace other forms of elitist disenfranchisement,” wrote Jay Lindsey, a Democrat, referring to the superdelegate system.

Yet even without superdelegates, Clinton maintains a more than 200-plus delegate lead over Sanders, whose candidacy now hinges on his winning as many delegates as possible in the upcoming contests in New York, Pennsylvania and California.

While many disagree with the superdeleagte system, Sanders supporters are not above playing the game if it will help the socialist Democrat win the nomination. Even Sanders’ campaign has acknowledged that it is working to “flip” superdelegates.

Last week, Sanders supporter Spencer Thayer set up a “superdelegate hit list”, a website dedicated to helping his fellow supporters prod superdelegates to switch their allegiance to the Vermont senator. And an online petition asking superdelegates to align their choice with voters rather than party elites has gathered more than 207,000 signatures.

Voter anxiety over process is nothing new but it is particularly sharp this election year, noted political scientist David Karol, a professor at the University of Maryland.

“When races are more competitive, the rules matter more,” Karol said. “In most cases, it’s not such a close contest and the rules don’t matter as much. This election has been different.”

Karol said voters often don’t understand the origins of the modern primary process, which evolved from a time when party leaders selected the nominee at political conventions. In those days, he said, candidates like Trump and Sanders wouldn’t have had a remote chance of being their party’s nominee.

That Trump is the Republican party’s clear frontrunner and Sanders, who only recently declared himself a Democrat, is locked in a competitive race with Clinton, the party’s once-presumptive frontrunner, is a testament to just how open the parties have become.

“America’s political parties are really more open and more participatory than they’ve ever been in the sweep of our history,” he said.