Democratic presidential candidate Bernie Sanders speaks during a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
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Democratic presidential candidate Bernie Sanders (lower centre) greets attendees after speaking at a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders speaks during a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders shakes hands with attendees before the start of a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
People cheer as Democratic presidential candidate Bernie Sanders takes the stage for a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders addresses a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. (Josh Edelson/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders addresses a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders addresses a rally at the Santa Clara County Fairgrounds in San Jose, California on May 18, 2016. (JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders (2nd-L) walks with his wife Jane Sanders (L) in San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders shakes hands with a man holding a beer while taking a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders poses for a photo while taking a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
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Democratic presidential candidate Bernie Sanders shakes hands with an unidentified man while taking a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
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Democratic presidential candidate Bernie Sanders shakes hands with a passerby while taking a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
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Democratic presidential candidate Bernie Sanders takes a walk between rallies in San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders shakes hands with a passerby while taking a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders speaks to supporters during a rally for local union members in San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders speaks during a rally for SEIU local union members in San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders speaks to supporters during a rally for local union members in San Francisco on May 18, 2016. (JOSH EDELSON/AFP/Getty Images)
US-VOTE-DEMOCRATS-SANDERS
Democratic presidential candidate Bernie Sanders poses for a photo with a group of street artists during a walk through San Francisco, California on May 18, 2016. / AFP / JOSH EDELSON (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Supporters cheer as Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Supporters cheer as Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Supporters cheer as Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)
Democratic Presidential Candidate Bernie Sanders Holds Campaign Rally In Bay Area
VALLEJO, CA - MAY 18: Democratic presidential candidate Sen. Bernie Sanders speaks at a campaign rally at Waterfront Park on May 18, 2016 in Vallejo, California. A day after winning the Oregon primary, Bernie Sanders is campaigning in California ahead of the state's presidential primary on June 7. (Photo by Justin Sullivan/Getty Images)