COLUMBIA, South Carolina (REUTERS) - South Carolina is at the epicentre of the Democratic presidential primary contest this weekend as 22 candidates descend on the state's capital to make their pitch for the party's nomination for president of the United States.

Candidates bounced between two events on Saturday across the street from one another in downtown Columbia that let candidates make their pitch to hundreds of voters.

South Carolina will be the fourth state to vote on a candidate for the party early next year, the first in which a significant proportion of the Democratic electorate - about 60 per cent - is black.

At the state Democratic Party's convention, a parade of more than 20 candidates appeared before organisers and activists in the party.

"Our strength is that we know how to organise, that we know how to do the critical work that needs to be done every day to ensure that we get our message across," said US Senator Kamala Harris.

There are 24 Democrats competing for their party's nomination to challenge Republican President Donald Trump in the November 2020 election.

The candidates in South Carolina offered remarks at a fish fry hosted by US House Majority Whip James Clyburn on Friday night, battling bugs and the sticky summer heat to shake hands with voters.

Their remarks were capped at just a minute, so the Saturday events offered a chance for more detailed exchanges.

Across the street from the party convention, Planned Parenthood Action Fund hosted a "We Decide 2020" forum.

As Republicans have stepped up efforts to limit access to legal abortion, the issue is increasingly galvanising Democrats.

"It is time to go on the offensive with Roe versus Wade, it's not enough to say we'll rely on the courts, we need to pass a federal law to make Roe versus Wade the law of the land," said US Senator Elizabeth Warren.

The forums gave even candidates who are struggling to build support in the polls an opportunity to speak and try to grow more backing ahead of next week's first primary debate.

"No one is asking you to decide who to vote for today, what I'm asking you to decide is who should be on the debate stage today," US Senator Kirsten Gillibrand said.

"Pick your top five and send them money, make sure they make it to the debate stage, this is not a time to be silent."

While the candidates stumped before their party's faithful, hopes of winning South Carolina in a general election remain thin.

South Carolina swings strongly Republican, and its governor and two US senators are Republican.

A Democrat last won the state in a presidential election in 1976, and Trump triumphed in South Carolina in 2016 with 54.9 per cent of the vote.