Despite Hillary Clinton having an almost unassailable lead in the delegates needed to clinch the Democratic presidential nomination, her opponent, Bernie Sanders, celebrated his primary victories and shifted his focus toward California, while speaking in San Jose on May 18. (Video/Reuters; Photo/Matt McClain, The Washington Post)

When Sen. Bernie Sanders of Vermont took the stage this week after falling short in the Kentucky primary, supporters of Hillary Clinton wondered whether he would finally soften his tone and let her move on to a general election against Donald Trump.

They didn’t have to wonder for long.

Sanders credited Clinton’s victory to “a closed primary, something I am not all that enthusiastic about, where independents are not allowed to vote.” He commanded the Democratic Party to “do the right thing and open its doors and let into the party people who are prepared to fight for economic and social change.” And then he promised that he’s staying in the race until the convention. “Let me be as clear as I can be: We are in ’til the last ballot is cast!”

The performance prompted cheers across a crowd of about 8,000 in Carson, Calif., highlighting the mistrust and alienation that Sanders’s most ardent fans feel about Clinton, the Democrats and their “rigged” system. Yet the whole spectacle also sent shudders through those supporting Clinton, who are growing increasingly irritated by Sanders’s ever-presence in the race — and nervous that he is damaging Clinton.

All of it seems to have come to a head in recent days, as bitterness on both sides has boiled over and prompted new worries that a fractured party could lead to chaos at the national convention and harm Clinton’s chances against Trump in November. Two realities seem to be fueling it all: The nomination is, for all intents and purposes, out of Sanders’s reach yet his supporters are showing no signs of wanting to rally behind Clinton.

“If you lose a game that you put your heart and soul into, and you lose squarely, you can walk off the court and shake someone’s hand and say, ‘Well done,’ ” said Rep. Diane Russell, a Maine legislator and Sanders supporter. “If you don’t feel like the game was working fairly, it’s hard to do that.”

On the other side is this view: It’s also hard to win a general election with a protracted, divisive primary battle that won’t go away. “The way he’s been acting now is a demonstration of why he’s had no support from his colleagues,” said former Massachusetts congressman Barney Frank.

Sanders supporters are crying “fraud” over delegate selection and threatening to sit out the election. They have promised to press their case to the convention floor. It happened in 2008, in the final throes of Clinton’s failed bid against Barack Obama. What remains unclear is whether this year’s divisions will go deeper or longer.

An explosive weekend convention in Nevada, where Sanders supporters turned on the state party chairwoman for overruling their challenges and seating Clinton delegates, exposed the depth of the acrimony. In his statements since then, Sanders has made no attempt to heal it.

Sanders is also keeping his supporters riled up by making what many Democrats view as an unrealistic, and even dishonest, view of his candidacy, given Clinton’s large lead in delegates.

“There are a lot of people out there, many pundits and politicians, they say Bernie Sanders should drop out, the people of California should not have the right to determine who the next president will be,” he said at Tuesday’s rally, insisting that the state had enough pledged delegates to put him over the top.

After a narrow victory in Oregon May17, Democratic presidential candidate Bernie Sanders is vowing not to give up in the fight against rival Hillary Clinton for his party's nomination. (Reuters)

Increasingly, Sanders’s most passionate supporters claim that the primary has been rigged. A Reddit user’s chart comparing the first wave of exit polls with Clinton’s stronger-than-expected performances has been circulated — most famously by Sanders surrogate and actor Tim Robbins — as evidence of election fraud.

Clinton’s 16-point victory in New York is explained by the state’s onerous registration rules and by the still-unexplained purge of Brooklyn voter rolls. Anyone questioning her lead of three million votes can find solace in a CounterPunch article titled “Clinton Does Best Where Voting Machines Flunk Hacking Tests.”

“Do these people read newspapers?” said Bob Mulholland, a California superdelegate and Clinton supporter who has accused Sanders supporters of harassing his peers. “Are they reading some chain email with bogus numbers? I hold Sanders somewhat responsible for this, because he comes across on TV as a very angry old man, riling people up.”

As Kentucky slid away from Sanders on Tuesday, some of his supporters saw a culprit in Alison Lundergan Grimes. The secretary of state and 2014 candidate for U.S. Senate, a longtime supporter of Clinton, even went on CNN to declare Clinton the winner.

“Hillary doesn’t even care anymore,” wrote one Sanders supporter, tweeting a link to a story about alleged fraud in Kentucky.

“Yet another state we would’ve won if everyone could vote,” another supporter wrote on Reddit.

“Better watch out for illegal conduct by Grimes since she said electing Clinton is more important than doing her job,” tweeted another.

The evidence for the last claim was a video clip from a rally with Clinton and Grimes, where the secretary of state said she was “not only here to do my job” but also to back her candidate. It was cut and distributed by America Rising, a conservative opposition research firm adept at finding wedges between Clinton and the left.

As Sanders has fallen behind Clinton, more conservatives have looked for ways to exploit the angst. On Tuesday morning, Fox News sent a morning-show host to the streets of New York to ask voters if the primary had been rigged for Clinton. Dan Backer, the conservative attorney and treasurer of the pro-Trump Great America PAC, has egged on Sanders supporters on Facebook with pep talks like “Bernie will win the most primaries and can still take the most pledged [elected] delegates while narrowing the total vote gap.” Trump has also announced a kind of snarky solidarity with Sanders, telling voters and Twitter followers that the senator should bolt the party over his foul treatment.

“Bernie Sanders is being treated very badly by the Democrats — the system is rigged against him,” Trump tweeted Wednesday morning. “Many of his disenfranchised fans are for me!”

The Sanders campaign has endorsed none of this — but it hasn’t tamped it down. Sanders’s sympathetic response to the Nevada convention fracas angered the state and national party, with DNC Chairwoman Debbie Wasserman Schultz comparing the worst scenes there to the violence at Trump rallies. Asked if there had been any actual fraud in the primaries, Michael Briggs, Sanders’s spokesman, suggested that the Democratic Party’s infrastructure had been sabotaged in a way that hurt one candidate.

“Most state parties tried to do a good job,” he said, “but often they are short on resources and there are institutional impediments to a fair process, like super-early registration, party-switch deadlines, closed primaries, complicated party registration rules, bad voter lists.”

Sanders himself has made harder-to-argue cases against the Democratic primaries. The truncated debate schedule struck supporters of both candidates as unfair, something the party seemed to acknowledge by tacking on more of them in March and April. Although Clinton is on track to win a majority of pledged delegates, Sanders has suggested that early support for Clinton among superdelegates, the party leaders and elected officials who get an automatic convention vote but are not bound by their state’s popular vote created a barrier no candidate could scale.

“It is absurd that you had 400 establishment Democrats on board Hillary Clinton’s campaign before anybody was in the race,” Sanders told MSNBC’s Rachel Maddow in an interview last week. “That stacks the deck in a very, very, unfair way for any establishment candidate, and against the wishes of the people.”

At the same time, Sanders and his supporters argue that superdelegates should consider bolting Clinton and backing him, based on polls that show him leading Trump as her favorables sink. That irritates Clinton supporters on two levels: by suggesting that the voters got it wrong and by dismissing the judgment of the sort of elected leaders whom any president would need to pass an agenda.

“If you believe you represent the people, and the people are uncooperative with your goal of winning, you have to find some explanation,” said Frank, whose appointment to the DNC rules committee sparked anger from Sanders’s supporters. “Look — I understand you have some disagreements, but does the overwhelming view of the black leadership, LGBT leadership, women’s leadership — does that count for nothing?”

As they contemplate Sanders’s “contested contest” at the Philadelphia convention, Clinton supporters think warmly back to 2008. By the time those primaries concluded, as many as 40 percent of Clinton voters said they could not support Barack Obama. The most dedicated PUMAs (Party Unity My A--) became TV stars; the vast majority of Clinton holdouts eventually went for the ticket. While Clinton’s favorable rating with Sanders supporters has been falling, many of his endorsers think that can be reversed.

“I want people to see this as a fair process, because I’m not in the ‘Bernie or Bust’ camp,” said Russell, the Sanders supporter from Maine. “I love this campaign, but I love my country more. And I tell the ‘Bernie or Bust’ people, if you’re angry at the end of this, you’re not going to take it out on the DNC. You’re going to take it out on the most vulnerable people — the ones we are fighting for.”