Democratic presidential candidate Sen. Bernie Sanders (I-Vt.) is rallying voters across battleground states against President Trump. (Video: Reuters, Photo: Sean Proctor/Reuters)

MADISON, Wis. — Sen. Bernie Sanders (I-Vt.) walked onstage here Friday under a gray sky with occasional specks of snow and launched into a full-on attack against President Trump, labeling him a “pathological liar” in a state that was key to Trump’s victory in 2016.

He repeated the epithet Saturday at a union hall in Michigan, another Trump state, and again on Sunday at a park in Pennsylvania, where Trump also won. “The biggest lie was that he was going to stand up for working families and take on the establishment,” Sanders said.

The blistering attacks on the president reflect Sanders’s developing, and arguably risky, strategy of reaching out to Trump’s voters — people the president has said would support him even if he shot someone. It’s a sharp contrast with other Democratic candidates who are focused on mobilizing Trump opponents. Not incidentally, it is also a way to signal to Democrats that Sanders is their best hope for knocking off Trump, at a time when many fear he is the opposite.

The most striking example of this strategy will play out Monday night when Sanders appears at a town hall meeting hosted by Fox News Channel, an outlet many Democrats detest and one the party has blocked from hosting a debate. Sanders says it’s important to talk to Fox viewers directly and tell them Trump misled them.

Sanders’s approach faces a significant test in Wisconsin, Michigan and Pennsylvania, where many white, working-class voters were drawn to Trump’s fiery populist message in 2016. Sanders argues that he can reclaim these voters by convincing them he can deliver the economic relief they were seeking all along.

But many Democrats across the country are unconvinced, even while they increasingly fear Sanders has a real shot at the nomination, given his solid base of support and the deeply fractured Democratic field, which has 18 candidates and counting.

Some worry that a national ticket led by a septuagenarian democratic socialist who wants to transform government will alienate the political center — not only helping Trump win a second term, but also erasing recent gains by centrist Democratic lawmakers in suburban areas.

If Sanders “wins the nomination, Trump will be president again,” said Rep. Kurt Schrader (D-Ore.). “I will guarantee it.”

Many of the Democratic gains in the 2018 election were made by candidates who were not in Sanders’s mold, including in two of the states he visited in recent days.

In Wisconsin and Michigan, Democrats nominated more traditional candidates for governor and captured both seats from Republicans. Some Democrats say a similar model is the path to victory in 2020.

Framing Sanders as the face of the Democratic Party, as the Fox appearance could do, will help Trump rather than hurt him, they contend.

The divide over Sanders reflects a dilemma at the heart of the Democratic primary: Should the party nominate a Democratic version of Trump who can match his combativeness, energizing liberals and taking the fight to the president? Or should it embrace a consensus builder, one who can rise above the country’s partisan anger and bring people together?

That question could take on even greater significance in coming weeks with the expected candidacy of former vice president Joe Biden. Biden’s bipartisan approach would sharpen the contrast between the field’s “unifiers” — Biden, Sen. Cory Booker (D-N.J.) and former congressman Beto O’Rourke — and its more aggressive partisans such as Sanders and Sen. Elizabeth Warren (D-Mass.).

Democratic concerns about Sanders echo some of the attacks Trump and his allies are deploying against the senator. Republicans have signaled that they would love to run against him, saying they could paint his policies as extreme and link him to Democrats up and down the ballot.

Trump’s comments about Sanders have been two-sided, simultaneously insulting him and building him up. Last year, Trump castigated the senator, whom he calls “Crazy Bernie,” for opposing the skimpier health insurance policies backed by Trump, tweeting, “Crazy Bernie and his band of Congressional Dems will outlaw these plans. Disaster!”

Then, as Sanders was entering the race earlier this year, Trump told reporters, “I like Bernie,” seeking to stoke complaints by Sanders’s loyalists that the Democratic Party mistreated him in 2016. “I think he was taken advantage of,” Trump said at an Oval Office signing ceremony. “I thought what happened to Bernie Sanders four years ago was quite sad.”

Sanders’s campaign manager, meanwhile, calls Trump a “faux Bernie Sanders” on trade, an area where both emphasize protecting American workers. At Sanders’s first campaign rally this year, an introductory speaker cited the “Crazy Bernie” label as a badge of honor.

At his Michigan rally Saturday, Sanders tore into Trump’s reworked version of the North American Free Trade Agreement, calling it weak on labor protections and urging the president to scrap it.

All this sets up an intriguing backdrop for Sanders’s Fox appearance. Fox is Trump’s favorite network, and he has a good relationship with some of its hosts. Sanders’s appearance on Fox to tell its viewers the president is a pathological liar could provoke one of the president’s outraged responses.

In his first two months as a candidate, Sanders has gotten off to a fast fundraising start, drawing large crowds and posting impressive showings in early polls.

Still, Sanders’s Midwestern rallies of recent days have not attracted the same massive crowds as during his kickoff events in New York and Chicago earlier this year. The audiences have shown some signs of animosity toward current and former rivals; Mark Craig, 66, disparaged Trump and Hillary Clinton with expletives as he cheered on Sanders during his speech Saturday in Warren, Mich.

The next phase of Sanders’s campaign, officials said, will be focused on activating a volunteer army of more than a million people and convincing voters that he is electable.

Campaign manager Faiz Shakir described this phase as “continuing to address what I think is one of the most critical questions in this race: Who is the best candidate to defeat Donald Trump?” He argued that Sanders is “uniquely positioned” to do so.

Sanders’s supporters say voters are deeply frustrated and looking for a leader who will shake things up. They note that 11 percent of general-election voters who picked Sanders in the 2016 Democratic primary went on to vote for Trump that November, according to the American National Election Studies survey. An additional 8 percent voted for minor-party candidates, and Sanders’s camp argues he has a shot with them as well.

Anita Cox, a 52-year-old teacher who attended the Sanders meeting in Indiana on Saturday wearing a “Feel the Bern” button, said she knows Trump voters whom she says Sanders could win over. “Maybe they believed his lies,” Cox said.

At a Sanders appearance in Michigan, Christina Fong, who supported Sanders in 2016, said she knows some people who voted for Sanders and then Trump. Sanders can win them back, she said.

“Unfortunately, a lot of times people are really angry, and sometimes they displace that anger,” said Fong, a 54-year-old musician from Grand Rapids. “So they remain angry, which he taps into,” she added, speaking of Sanders.

But many Democratic leaders are watching Sanders with concern, believing that the better bet for beating Trump is a less hostile messenger with a more moderate message. Nominating someone who neither personifies diversity nor connects with centrists is one of the few ways Democrats could lose in 2020, they argue.

“I don’t think anybody’s for basically socialism. I really don’t,” said Sen. Joe Manchin III (D-W.Va.), who won reelection last year in a state Trump captured decisively in 2016.

Sanders’s unapologetic embrace of a Medicare-for-all health plan that would effectively eliminate private insurance also worries some party operatives. Democrats won in 2018 by depicting GOP health plans as extreme, they say, and they have no desire to see the tables turned in 2020.

“I think when people find out that certain candidates want to take away people’s employer-sponsored health care, that’s going to be very worrisome for some Democrats, especially in some of these affluent suburbs,” said Ian Russell, former deputy executive director of the Democratic Congressional Campaign Committee, which coordinates the party’s House campaigns.

Sanders campaigned for Democrats in the Midwest several times in the run-up to the 2018 midterms, but most of his endorsed candidates lost. Brent Welder of Kansas and Pete D’Alessandro of Iowa, for example, were defeated in their House primaries by more moderate Democrats who went on to win in November.

Sen. Doug Jones (D-Ala.), who faces reelection in a ruby-red state, said his constituents want a consensus builder at the top of the ticket, “someone who can govern, who can reach across, not only across the aisle, but within the various groups, within the Democratic Party.” He declined to say whether Sanders qualifies.

In many ways, the Democrats’ current dynamic is similar to the Republican landscape in 2016, when Trump prevailed: a party unsure how to deal with a rowdy populist from outside its traditional ranks, a crowded field that plays to his advantage, and an unpredictable political climate.

Republicans, for their part, say they are eager to run against Sanders.

“Speaking for North Carolina, if America had a choice between a self-avowed socialist democrat and a free-market capitalist, he loses,” said Sen. Thom Tillis (R-N.C.), who is up for reelection. “Period, end of story.”

Of course, Democrats said the same thing about Trump in 2016. And many in the party are signaling that they recognize the political appeal of Sanders’s positions.

When Sanders unveiled his Medicare-for-all bill Wednesday, he had 13 co-sponsors, including four of his presidential rivals. One of them, Sen. Kirsten Gillibrand (D-N.Y.), appeared at a news conference with Sanders.

In another echo of the Trump 2016 phenomenon, Sanders’s rivals, for now at least, are being careful what they say about him, wary of starting a feud and attracting the wrath of his followers.

As she left the Capitol on Wednesday, Warren, who is also seeking the nomination, agreed that Sanders has gotten off to a fast start. But when asked why, she demurred.

“I’m sorry, but I am late,” she said as she walked briskly away. A few seconds later, she added, “He’s doing great.”

The limits of Sanders’s capacity to win Trump voters came into focus Sunday at a town hall in Warren, Ohio. The event took an abrupt turn when a man moved toward the senator shouting “Go Trump!” and “You’re a socialist!” before he was physically removed by security.

David Weigel and Scott Clement contributed to this report.