Bernie Sanders and Hillary Clinton are almost tied in the polls as they head into the California Democratic primary election on 7 June. According to an NBC/Wall Street Journal/Marist poll, Clinton has only a 2 point lead of 49, to Sander's 47.

Clinton is hoping to secure the Democratic nomination as she needs just less than 30 of the 548 delegates available in California.

"This is a real nail-biter of a race," said political consultant Dave Jacobson. "Nobody expected California to be a close, battleground contest, but this is really the last stand for Bernie Sanders, in terms of the optics, right?"

Sanders has been reported to have been visiting various towns in advance of the election, but Jacobson said: "When it comes to the arithmetic, there's no viable pathway for him to lock up the Democratic nomination. Hillary Clinton only needs eight percent of the delegates moving forward to hit the critical 2,383 figure.

"And so at this point for Bernie Sanders, it's more about creating a movement and building some momentum going into the Democratic party convention, to sort of glue his narrative and his platform to the convention with the hopes that some of the policies that he's campaigned on will be attached to the Democratic party platform."