More than half of American voters believe that the system U.S. political parties use to pick their candidates for the White House is “rigged” and more than two-thirds want to see the process changed, according to a Reuters/Ipsos poll.

The results echo complaints from Republican front-runner Donald Trump and Democratic challenger Bernie Sanders that the system is stacked against them in favor of candidates with close ties to their parties – a critique that has triggered a nationwide debate over whether the process is fair.

The United States is one of just a handful of countries that gives regular voters any say in who should make it onto the presidential ballot. But the state-by-state system of primaries, caucuses and conventions is complex. The contests historically were always party events, and while the popular vote has grown in influence since the mid-20th century, the parties still have considerable sway.

One quirk of the U.S. system – and the area where the parties get to flex their muscle – is the use of delegates, party members who are assigned to support contenders at their respective conventions, usually based on voting results. The parties decide how delegates are awarded in each state, with the Republicans and Democrats having different rules.