o understand Bernie Sanders’ unwillingness to concede the Democratic presidential nomination race to Hillary Clinton, take a look at some of the Twitter hashtags used by his supporters:

#StillSanders

#BernieOrBust

#SeeYouInPhilly

Clinton has effectively locked up the nomination, but Sanders continues to say he’ll take his fight to transform the Democratic Party — and the country — all the way to the party's national convention in Philadelphia in July. His supporters, it seems, will accept nothing less.

“We will never stand down,” tweeted “Women for Bernie” on Sunday. “It’s not over.”

Some polls in recent months show anywhere from about 25% to 50% of Sanders backers weren’t ready to support Clinton either as the nominee or in a matchup with the presumptive GOP nominee Donald Trump.

Lee Miringoff, director of the Marist Institute for Public Opinion in New York, said more Sanders voters likely will shift to Clinton as a “thaw” between the two candidates takes hold. That process will be guided partly by the signals Sanders sends to his supporters, he said.

“He has continually said that the worst thing would be for Trump to win,” Miringoff said. “Given that he understands the math, that sort of means that he has to support Hillary Clinton.”