There was a danger sign for Hillary Clinton in a dank old industrial warehouse here along the Milwaukee River. The cinder-block walls had been painted over with modernist murals bearing the likeness of Bernie Sanders. This was now a campaign field office. And the volunteers, young and old, who manned a phone bank Thursday afternoon vowed never to give up their time — or, for some, even their votes — to Clinton.

Would Teresa VanDoorn, 44, a homemaker who had become a familiar face at the Sanders office, support Clinton if she became the Democratic presidential nominee?

“No,” VanDoorn said. “Voting for Hillary would be approving of the status quo and establishment — and I don’t approve of that. I would write Bernie’s name in. I consider Hillary equal to the GOP candidates, to be frank.”

What about Lily Shea, 19, who has been helping Sanders ahead of Tuesday’s Wisconsin primary? “No, I would write Bernie in — definitely,” she said. “I just don’t trust her. Whenever I hear her talk, I get the feeling there’s something else going on behind the curtains.”

Then there was Patrick Doyle, 55, a Sanders true believer. Asked if he could shift his allegiance to Clinton, Doyle paused to think, then said flatly, “I’m going to do everything I can to make sure that doesn’t happen.”

The Fix's Aaron Blake breaks down what's at stake for the GOP candidates in the April 5 Wisconsin primary. (Peter Stevenson/The Washington Post)

Clinton is well on her way to the nomination. But Wisconsin — a fall battleground with a celebrated tradition of progressive activism and political reform — represents a phenomenon that could undermine her in the general election: She has yet to energize some parts of the liberal base or even persuade them to be comfortable with her candidacy.

Whether she prevails in Wisconsin’s primary or, as polls suggest is more likely, Sanders edges her here, Clinton would maintain her sizable delegate lead. To overtake her, Sanders would need blowout wins in many of the states still to vote, including New York, Pennsylvania and California.

General election polling indicates Clinton would easily defeat Republican front-runner Donald Trump in Wisconsin and other swing states. The data also suggests that the majority of Sanders supporters would vote for Clinton.

Yet among Sanders’s most fervent backers — the committed liberal activists who would be necessary in any grass-roots effort in a general election and whose votes could be decisive if the race tightens — there is a strong distaste for Clinton. To them, she represents the ­moneyed, special-interest politics they have been fighting against.

Actress Susan Sarandon, a Sanders surrogate, gave voice to this view when she said on MSNBC a few days ago that she would consider sitting out the election if Clinton is the nominee and that a Trump presidency, in the minds of some Sanders fans, might bring about a more immediate “revolution.” Sanders campaign manager Jeff Weaver later distanced the campaign from Sarandon’s comments and said Sanders would support the Democratic nominee.

This hostility has stirred hard feelings in the Clinton orbit. At Clinton’s Milwaukee field office on Thursday, Sen. Cory Booker (D-N.J.) gave a pep talk to volunteers in which he testified to the former secretary of state’s competence, resiliency, tenacity and grit.

As Booker wrapped up, Winifred Thrall, a 78-year-old Clinton backer, cried out, “Cory! I wish that Susan Sarandon could have heard you.”

In an interview later, Thrall bemoaned that so many Wisconsin progressives are backing a candidate, Sanders, who in her view “has no experience — nothing but calling for a revolution in the streets.”

“It’s so upsetting,” Thrall said. “I have an old friend — we worked on McCarthy and McGovern together, in Appleton, Wisconsin — and I can’t talk to her anymore because of her support of Bernie.”

Wisconsin’s primary is important because, in many respects, the state is a microcosm of the Democratic Party nationally and has an unusually engaged electorate.

“We’ve got industrial, urban, rural, small towns, colleges, high tech — it’s all here,” said Democratic former governor Jim Doyle, a Clinton supporter. “Eight years ago, it was a big deal when [Barack] Obama came in and beat Hillary Clinton badly here.”

Tad Devine, Sanders’s chief strategist, said Wisconsin is “a great proxy for a candidate’s strength in the general election. This is a state that Democrats must have.”

The primary is competitive. This week’s Marquette Law School survey, considered the state’s gold standard, had Sanders leading 49 percent to 45 percent. The poll shows that Sanders is leading 57 percent to 37 percent among self-identified independents — part of an alarming national trend for Clinton of being unpopular with unaffiliated voters who can help swing general elections.

“It will be critical for the nominee to be able to win substantially among people who lean Democratic and younger voters, and it does leave the question of whether, absent Bernie Sanders, could Hillary Clinton really rack up large margins,” said Charles Franklin, a professor who oversees the Marquette Law poll.

Joe Zepecki, a Wisconsin-based communications consultant who is not working for either campaign, said independent ­voters pose a big challenge for Clinton. “I don’t think they’re going to waltz right over,” he said.

Sanders has an enthusiastic base in liberal Dane County, home to the University of Wisconsin at Madison, and he also has found support in rural farming areas and manufacturing towns. He is trying to tap into the state’s tradition of political reform; one of his ads features a dairy farmer from his home state of Vermont testifying to Sanders’s independence from special interests.

Barbara Lawton, a former lieutenant governor who backed Clinton in 2008 and is supporting Sanders now, said the Wisconsin electorate is “deeply unsettled” — in part because of now-chronic wage stagnation. She said there was a “tone deafness” to Clinton’s campaign, given her ties to Wall Street and support for trade deals, that prevents her candidacy from resonating more with progressive Democrats.

“We’re really looking for someone who you can trust,” Lawton said. “There’s such a longing for the truth.”

Much of Clinton’s support is concentrated in the largest city, Milwaukee, which has a sizable black population, and suburban areas. Booker, the Senate’s only black Democrat, spent Thursday trying to energize Clinton supporters in this city’s neighborhoods.

“What happens in this primary in just a few days is going to affect the trajectory of this campaign. It’s going to affect the spirit of what’s to come,” Booker told a few dozen volunteers at Clinton’s storefront office on Dr. Martin Luther King Drive.

Both candidates have been advertising here and are fanning out across the state this weekend. The Clinton campaign is trying to maximize her performance in targeted congressional districts to ensure that, even if Sanders wins statewide, the two will roughly split Wisconsin’s 86 pledged delegates.

“What we’re seeing this year is so classic: There’s the establishment candidate and the liberal challenger,” Doyle said. “There’s a long history of progressive, liberal candidates making their last stands here.”

Clinton allies rejected any suggestion that a loss to Sanders in Wisconsin would reveal weaknesses that could haunt her in a general election.

“The word ‘weakness’ deters any kind of response from me as it relates to Hillary. There are no weaknesses,” said Martha Love, a longtime African American leader from Milwaukee and a member of the Democratic National Committee. “Our state will fall in line behind the Democratic candidate because the other side — my gosh, my gosh, my gosh, my gosh.”

Matt Flynn, a former state party chairman and a Clinton booster, echoed that view: “If she loses Wisconsin and people are lackluster and aren’t showing any passion about her, does it mean she’ll struggle in the general election? No, because she is running against two of the biggest freaks in the history of American politics: Trump and [Sen. Ted] Cruz.”

At Clinton’s Milwaukee office, volunteers said they were not worried about Sanders backers eventually joining their effort. Some said it may not take any persuasion at all.

Jill Huennekens, 46, a bartender who called Clinton “my hero,” said she has been making phone calls on Clinton’s behalf for months.

“Truthfully, I can’t remember one that’s been Bernie or bust,” she said. “They tell me that they will support Hillary in November, even if they want Bernie in the primary. They say it even without my asking — ‘Don’t worry, she has our support in November.’ ”

Catherine Wolfe, 59, another Clinton volunteer, said the message she tells fellow Democrats is simple: “For those people looking for a ‘revolution,’ all they have to do is vote for a woman, and we’ll have a revolution.”