Senator Bernie Sanders of Vermont beat Democratic presidential front runner Hillary Clinton in the West Virginia primary on Tuesday, showing he still has the support of white, working-class Americans - a critical demographic that the Clinton campaign has failed to win over.
Meanwhile, Republican presumptive nominee Donald Trump, who ran uncontested on Tuesday, easily swept both the West Virginia and Nebraska primaries.
Mr Trump is in the midst of short-listing his running mate, according to US news outlets.
Observers say Mrs Clinton's loss in West Virginia is just a small bump on the road, as they believe she will get the required 2,383 delegates to become the party's nominee in July.
Mrs Clinton has 2,239 delegates, who include pledged and super delegates - typically party officials - while Mr Sanders has only 1,469.
Assistant professor of political science Damien Arthur at Marshall University (MU) in West Virginia said: "Her delegate lead is too great at this point. I know Sanders is touting a momentum narrative, but the maths just does not support that, even if the super delegates change their minds, which they won't."
Mr Sanders' strategy is to win a majority of pledged delegates in a bid to sway super delegates over to his camp.
He said in his victory speech on Tuesday night: "Despite all the opposition, we have now received over 45 per cent of the pledged delegates and if we do well in the coming weeks... we still have that road to victory in winning the majority of pledged delegates."
While Mr Sanders acknowledged his campaign's "uphill climb", he reiterated his commitment to stay in the race till the very end.
"We have now won primaries and caucuses in 19 states, and let me be as clear as I can be, we are in this campaign to win the nomination and we are going to fight for every last vote in Oregon, Kentucky, California," he said.
His recent wins in Indiana and West Virginia, however, are unlikely to erode Mrs Clinton's popularity in the upcoming delegate-rich states such as New Jersey and California, said experts.
"Clinton is leading in both states and Democratic voters in these states are probably more typical of the 'establishment' Democrats that tend to support Hillary Clinton," said associate professor George Davis, chair of the political science department at MU.
As for why Mrs Clinton was unable to win over West Virgina voters, Dr Davis boiled it down to distrust of the former secretary of state and the fact that many parts of West Virginia are still lagging behind in economic growth and job creation.
"Sanders is speaking directly to those issues; therefore a considerable number of West Virginian Democrats, especially younger voters, are attracted to him," Dr Davis said.