Bernie Sanders is acknowledging that his path to the Democratic nomination to run for the White House depends on flipping superdelegates, the party insiders who can back either candidate and are overwhelmingly behind rival Hillary Clinton.
It is a surprising admission by the Democratic presidential candidate, who formally joined the party a year ago.
He called on superdelegates to cast their votes "in line with the people of their states".
But he also noted that even if superdelegates from states Mr Sanders won flipped to his side, he would still face a narrow path to the nomination.
Mrs Clinton is still 91% of the way to the nomination, according to the Associated Press.
She is 218 delegates away from winning the 2,383 need to clinch the nomination.
"We have an uphill climb no question about it," he said.
Mr Sanders spoke at a news conference in Washington before heading to campaign events in Indiana.
Former President Bill Clinton drew boos and shouts from the crowd as he made a campaign stop in Logan, West Virginia, on his wife's behalf, ahead of the state's May 10 presidential primary.
Supporters of Mr Sanders and Republican candidate Donald Trump gathered outside the school as Mr Clinton spoke on Sunday.
According to WVNS-TV, a letter written on behalf of Logan officials told US Senator Joe Manchin's staff in an email that Mr and Mrs Clinton "are simply not welcome in our town".
Mrs Clinton, who planned to campaign in Williamson on Monday, has been criticised for comments that her policies would put coal miners and companies out of business. Mrs Clinton said later she was mistaken and that she is committed to coalfield communities.
Bill Clinton planned to attend a rally later in Charleston.
Mr Trump said that, if he becomes the GOP presidential nominee, he will raise money for Republican Senate candidates.
He said on Sunday in Indiana that competing against republican rivals is "wasting time" he could be spending raising campaign cash.
Mr Trump has frequently boasted that his presidential campaign is self-funded. While Trump has not held traditional fundraisers and lacks a financial team to solicit checks, his campaign website features a large, red "donate" button.
Mr Trump said Republicans "have fallen out of love" with his main rival, Ted Cruz.