Pressure is building on Vermont Senator Bernie Sanders to give up his fight for the Democratic presidential nomination.

Hillary Clinton has been declared the presumptive Democratic nominee after winning four out of six primaries on Tuesday.

Sanders has virtually no path to the nomination but has vowed to fight on. He meets President Obama on Thursday.

Democrats have urged Sanders to join Clinton as she tries to defeat Donald Trump.

The billionaire businessman, seen as an outsider early in the campaign, is the Republicans' presumptive nominee.

The White House meeting on Thursday came at Sanders' request. He will hold a rally in Washington the same day ahead of the District of Columbia's primary on 14 June.

He has won 22 state primaries and caucuses but failed to make a significant dent in Clinton's delegate lead, despite mounting what has been seen as an impressive campaign, influencing the Democratic debate and engaging with thousands of young voters.

"The struggle continues. We are going to fight for every vote in Tuesday's primary in Washington DC, and then we will bring our political revolution to the Democratic convention in Philadelphia," his campaign said in a statement on Wednesday.

As the votes were being counted in California, Sanders said: "I am pretty good at arithmetic, and I know that the fight in front of us is a very, very steep fight, but we will continue to fight for every vote and every delegate we can get."

Going into the California primary, Sanders said he hoped to flip superdelegates, who do not have to say who they are supporting until the party convention in July, to come to his side.

But many political commentators have cast doubt on that aspiration.

Vice President Joe Biden has said it is up to Sanders to decide when to drop out.

"Oh, let him make that decision," he said. "Give him time."

Senator Bill Nelson from Florida said Sanders should "stand down" and Senator Bob Casey said Democrats must come together and unify.

Senator Jeanne Shaheen from New Hampshire said Sanders should end his campaign soon.

The Sanders campaign is downsizing its staff by about half ahead of next week's DC primary.