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.

The delegates' personal opinions can come into play at the party conventions if the race is too close to call - an issue that has become a lightning rod in the current political season.

Another complication is that state governments have different rules about whether voters must be registered as party members to participate. In some states, parties further restrict delegate selection to small committees of party elites, as the Republican Party in Colorado did this year.

'SO FLAWED'

"I'd prefer to see a one-man-one-vote system," said Royce Young, 76, a resident of Society Hill, South Carolina, who supports Democratic front-runner Hillary Clinton. "The process is so flawed."

    Trump has repeatedly railed against the rules, at times calling them undemocratic. After the Colorado Republican Party awarded all its delegates to Ted Cruz, for example, Trump lashed out in a Wall Street Journal opinion piece, charging "the system is being rigged by party operatives with 'double-agent' delegates who reject the decision of voters."

Republican National Committee Chairman Reince Priebus has dismissed Trump's complaints as "rhetoric" and said the rules would not be changed before the Republican convention in July.

Trump swept the five Northeastern nominating contests on Tuesday in Pennsylvania, Maryland, Delaware, Connecticut and Rhode Island. The New York billionaire has 950 delegates to 560 for Cruz, a U.S. senator from Texas, and 153 for Kasich, the Ohio governor, according to the Associated Press. A total of 1,237 delegates are needed to secure the Republican nomination.

On the Democratic side, Sanders, a U.S. senator from Vermont, has taken issue with the party's use of superdelegates, the hundreds of elite party members who can support whomever they like at the convention and who this year overwhelmingly back front-runner Hillary Clinton.