Joe Biden, Donald Trump win Kentucky, Oregon primaries

US President Joe Biden and his Republican rival Donald Trump have piled up more delegates as both presumptive nominees won primaries in Kentucky and Oregon.

The symbolic decisions provide a few more delegates to the national conventions and a gut check on where the Democratic and Republican bases stand toward their standard-bearers as the presidential nominating season nears its end.

Even after they secured the nominations and their rivals dropped out, Biden and Trump have continued facing dissent from within their parties.

Biden has faced protest votes over his handling of the Israel-Hamas war while Trump is still seeing thousands of people voting for long-vanquished rival Nikki Haley.

That trend continued on Tuesday in Kentucky with about 18 per cent of the Democratic vote going to "uncommitted" with roughly 80 per cent of the vote counted. In the GOP race, Haley was winning about six per cent.

After Tuesday, eight presidential nominating contests will remain: Democrats in Idaho, the District of Columbia, Guam and the Virgin Islands, and both parties in Montana, New Jersey, New Mexico and South Dakota.

Voters in Kentucky, Oregon, Georgia and Idaho also held state primaries Tuesday to choose nominees for the US House and other contests.

In California's Central Valley, Vince Fong, a California State Assembly member backed by Trump, won a special election to complete the remainder of the term of deposed former US House Speaker Kevin McCarthy.

Fong defeated fellow Republican and Tulare County Sheriff Mike Boudreaux.

In Georgia, Fulton County District Attorney Fani Willis, who is leading the prosecution of Trump in a 2020 election interference case, defeated challenger Christian Wise Smith in the Democratic primary. The judge in the case, Scott McAfee, also won his election.

In Oregon's Multnomah County, home to Portland, the progressive district laywer who took office during the social justice movement of 2020 is being challenged by a candidate vowing to be tough on crime.