Former Secretary of State Hillary Clinton won the Democratic primary in Kentucky, while Senator Bernie Sanders won in Oregon - leaving the front runner now within touching distance of the delegate count she needs, even as Sen Sanders pledged to battle on.

The latest estimates by the Associated Press put Mrs Clinton's delegate count at 2,289, a number that includes 524 party elites known as superdelegates. She needs just 94 delegates to reach an outright majority and seal the nomination. That will now almost certainly happen on June 7 when a series of large states like California and New Jersey go to the ballot.

In effect, the split night does little for either candidate.

For Mrs Clinton, the narrow victory in Kentucky and the loss in Oregon highlights the continued weakness in her campaign among younger voters and the American working class. For Mr Sanders, a loss - even a narrow one - dents his momentum and makes the already improbable task of overhauling Mrs Clinton's lead near impossible.

The senator, however, showed no signs of giving up. In fact, he appears to be taking an increasingly defiant tone. At a rally in California, he appeared to downplay widely agreed-upon analyses that he does not have a plausible path to victory.

He needs to win nearly 70 per cent of all remaining delegates to pip Mrs Clinton in the number of pledged delegates.

So far, however, he has won only about 45 per cent of delegates and there are no signs that he will enjoy a sudden surge of support.

"It will be a steep climb, I recognise that, but we have the possibility of going to Philadelphia with a majority of the pledged delegates," he told the crowd. "Together we have been climbing that steep hill from day one in this campaign. And we are going to continue to fight for every last vote until June 14 and then we are going to take the fight into the convention."

Mr Sanders also showed no signs of heeding calls from Democratic Party leaders to rein in attacks on Mrs Clinton that could work in favour of presumptive Republican nominee Donald Trump.

"No one can predict the future, but I think we have a real shot to win primaries in a number of the states that will be coming up. And don't tell Secretary Clinton because she might get nervous, I think we're going to win here in California," he said to a cheering crowd.

Earlier, he questioned Mrs Clinton's judgment for saying she would put her husband, former president Bill Clinton, in charge of revitalising the economy and refused to apologise for the behavior of his supporters at a recent Nevada convention.

Supporters of the senator had sent the state's party chair death threats and insults because they felt the proceedings had unfairly favoured Mrs Clinton.

Mr Sanders instead issued a challenge to the Democratic Party leadership.

"The Democratic Party has a choice. It can open its doors and welcome into the party people who are prepared to fight for real economic and social change - people who are willing to take on Wall Street, corporate greed and a fossil fuel industry which is destroying this planet.

"Or the party can choose to maintain its status quo structure, remain dependent on big-money campaign contributions and be a party with limited participation and limited energy," he said in a statement that he also repeated to the crowd in California.

The souring relationship between the two candidates has Democrats increasingly worried that it would make party unity difficult heading into the general election. Mr Trump has also indicated he intends to use Mr Sanders' attacks as a playbook on how to tackle Mrs Clinton.

The real-estate mogul easily won the Republican primary in Oregon as he is the only active candidate in the party.

After today, there are only nine more contests in the Democratic Party race.