More than half of American voters believe the system US 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.

Key points: 71 per cent would like to pick their nominee with a direct vote and cut out delegates

71 per cent would like to pick their nominee with a direct vote and cut out delegates Over a quarter Americans don't understand the primary process

Over a quarter Americans don't understand the primary process Analysts say 'arcane rules' come into play only during tight races

The results echo complaints from Republican front-runner Donald Trump and Democratic challenger Bernie Sanders that the system is stacked against them in favour 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.

Larry Sabato, director of the University of Virginia Centre for Politics, said the US presidential nominating system could probably be improved in several areas, but noted the control wielded by party leadership usually becomes an issue only during tight races.

"The popular vote overwhelms the rules usually, but in these close elections, everyone pays attention to these arcane rules," he said.

Some 51 per cent of likely voters who responded to the April 21-26 online survey said they believed the primary system was "rigged" against some candidates.

Some 71 per cent of respondents said they would prefer to pick their party's nominee with a direct vote, cutting out the use of delegates as intermediaries

The results also showed 27 per cent of likely voters did not understand how the primary process works and 44 per cent did not understand why delegates were involved in the first place — the responses were about the same for Republicans and Democrats.

Overall, nearly half said they would also prefer a single primary day in which all states held their nominating contests together — as opposed to the current system of spreading them out for months.

The poll included 1,582 Americans and had a credibility interval of 2.9 percentage points.

'The process is 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."

One quirk of the US 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.

Reuters