A big win in the 7 June California primary could hand Mr Sanders hundreds more delegates, which would call into question Ms Clinton's candidacy

Hillary Clinton could lose the race for Democratic nominee as the focus shifts to California, where an influx of voter registrations threatens to derail her lead over Bernie Sanders.

Mr Sanders will be looking to the state to boost his campaign with news of a further 1.5 million people registering to vote since January this year.

The latest statistics from the Institute of California will be encouraging to Mr Sanders as a big win in the 7 June California primary, where the candidates are currently virtually deadlocked, could hand him hundreds more delegates. Mr Sanders currently has 1,501 pledged delegates to Ms Clinton's 1,769.

The influx of additional registrants – a 218 per cent increase compared with the same period in 2012 – is likely to include large numbers of young voters and could make this scenario a reality.

Sanders Criticises 'Super Delegate System'

Recent open primaries have shown that the Vermont senator tends to underperform in pre-election surveys and over-perform on primary and caucus days, thanks to the participation of new registrants and young voters.

If Mr Sanders were to win in California it would seriously call into question Ms Clinton's candidacy in the general election and could result in a number of superdelegates, including distinguished party leaders and elected officials who are free to support any candidate for the presidential nomination, withdrawing their support.

Mr Sanders said although Ms Clinton has received "a whole lot more" superdelegate support than he has to date, "they don’t vote until they're on the floor of the Democratic convention". He said that his job was now to convince them of his electability against Donald Trump.

In pictures: US Elections 2016

He is considered the stronger contender against Mr Trump nationally, with recent surveys putting him ahead by about 10 points.

Mr Trump and Ms Clinton, currently subject to a damning report on misuse of a private email server while she was secretary of state, are possibly the most disliked nominees in decades, while Mr Sanders is the candidate more liked than disliked in favourability ratings.