Hillary Clinton had hoped to have long ago sealed the Democratic Party's presidential nomination, but challenger Bernie Sanders has proven a formidable foe, and the battle between the candidates turns to the east-central US state of Kentucky and north-western state of Oregon on Tuesday.

Sanders, who has had strong showings in the most recent contests, has vowed to continue in the state-by-state primary process in the hopes of forcing the party's nominee to be chosen at the party convention in July.

There are few opinion polls to provide a window into the candidates' chances in Kentucky and Oregon.

Clinton has strong ties in Kentucky, where she won the 2008 primary versus Barack Obama and where she has powerful political allies dating to the time of husband Bill Clinton's presidency in the 1990s.

However the former US secretary of state lost to Sanders in neighbouring West Virginia after negative comments about the region's coal industry that could also damage her chances in eastern Kentucky. Sanders' economic message could also resonate among the state's working-class and young voters.

Oregon is known as a liberal bastion on the US West Coast that could be fertile ground for Sanders, a US senator from Vermont and a self-described democratic socialist, who won by large margins in neighbouring Washington state in March.

Oregonians cast their ballots by mail, and the state is on track for a record turnout of more than a million votes, the Oregonian newspaper reported.

Sanders' campaign has argued that a high voter turnout favours their candidate, but a poll this week in the state favoured Clinton.

"Oregon is one of the most progressive states in the United States, and the agenda we have is the agenda that the people of Oregon feel comfortable with," Sanders told the newspaper.

There are 61 Democratic delegates at stake in Kentucky and 74 in Oregon.

Clinton has a strong lead in the total number of delegates needed to secure the nomination with 2,240 total to Sanders' 1,473, according to a tally kept by the New York Times. Clinton's delegate however is bolstered by support from party officials who may vote for whomever they chose, and Sanders hopes to peel away some of that support if he can continue to win upcoming contests.

On the Republican side, Donald Trump is the party's presumptive nominee after his remaining challengers left the race earlier this month. Republicans in Kentucky gave Trump the nod in a party caucus in March and therefore won't be voting on Tuesday, but Republicans in Oregon will have their first chance to weigh in on Tuesday.

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