US Democratic presidential candidate Bernie Sanders has defeated Hillary Clinton in West Virginia's primary, winning over voters deeply sceptical about the economy and signalling the difficulty Clinton may have in industrial states in the general election.
The loss slows Clinton's march to the nomination, but she is still heavily favoured to become the Democratic candidate in the November 8 election.
In a November match-up with Donald Trump, Clinton will need to win over working-class voters in the US Rust Belt, which includes key states such as Ohio and Pennsylvania.
Trump, 69, won contests in West Virginia and Nebraska handily on Tuesday (local time). The presumptive Republican nominee is set to meet with party leaders in the US Congress on Thursday, including US House of Representatives Speaker Paul Ryan.
After Ryan said last week that he was not yet ready to endorse Trump, Trump said on Sunday that he would have to decide whether he still wanted Ryan to preside over the party's July convention.
Trump said in a Fox interview on Tuesday night that he would like Ryan to chair the convention as planned. "He's a very good man, he wants what's good for the party," the New York billionaire said.
Trump has zeroed in on Clinton's protracted battle with Sanders, a 74-year-old US senator from Vermont. He has taunted Clinton in recent days by saying she "can't close the deal" by beating Sanders, her only rival for the Democratic Party's nomination since February 1.
Clinton, 68, has said she will ignore Trump's personal insults, including his repeated use of his new nickname for her, "Crooked Hillary", and instead will criticise his policy pronouncements.
Deep concerns about the economy underscored West Virginia's Democratic primary. Roughly six in 10 voters said they were very worried about the direction of the US economy in the next few years. The same proportion cited the economy and jobs was their most important voting issue, according to a preliminary ABC News exit poll.
A remark Clinton made at an Ohio town hall in March that the country would "put a lot of coalminers and coal companies out of business" may have hurt her with voters in coalmining states such as West Virginia.
During Clinton's visit to West Virginia and Ohio last week she repeatedly apologised to displaced coal and steel workers for her comment, which she said had been taken out of context, and discussed her plan to help retrain coal workers for clean energy jobs.
To secure the Democratic nomination, a candidate needs 2383 delegates. Going into West Virginia, Clinton, a former US secretary of state, had 2228 delegates, including 523 so-called superdelegates, elite party members who are free to support any candidate.
Sanders had 1454 delegates, including 39 superdelegates. Another 29 delegates will be apportioned based on West Virginia's results.
Clinton and Sanders will compete in another primary contest on May 17. Both candidates are also looking ahead to the June 7 contests, the last in the long nominating season, in which nearly 700 delegates are at stake, including 475 in California, where Sanders is now focusing his efforts.
Sanders has vowed to take his campaign all the way to the Democrats' July 25-28 convention in Philadelphia, and wants a say in shaping the party's platform.
Sanders has repeatedly told supporters at packed rallies that most opinion polls indicate he would beat Trump in a general election match-up by a larger margin than polls show Clinton defeating Trump.
Trump, shifting into general election mode, has already begun to consider running mates. He told Fox on Tuesday night that he has narrowed his list to five people.
He did not rule out picking New Jersey Governor Chris Christie, a former rival who ended his presidential bid in February. Christie, who endorsed Trump and then campaigned for him, on Monday was named to head Trump's White House transition team.