The past few weeks have put on remarkable display just how far to the left the Democratic Party has moved, with many in the 2020 presidential field now embracing previously untouchable positions on health care and immigration.

Democrats once touted their defense of those with preexisting conditions — a stance supported by the vast majority of Americans — but many leading presidential candidates now support ending the private insurance coverage on which most of the country relies. Democrats used to focus almost exclusively on reuniting migrant children and their families and protecting undocumented immigrants brought here by their parents; now many of the candidates are openly espousing making illegal border crossing a civil offense rather than a criminal one.

The high-profile shifts on issues that voters say they care about most mark a risky gambit by many candidates to tap into new energy in the party’s liberal base, the home of some of its loudest voices in the primary season. But it is triggering new worry that it comes at a cost: confirming Republican arguments that Democrats are far out of the mainstream, a threatening posture when moderate suburban voters have been the linchpin to winning general elections and down-ballot races.

“Donald Trump, you can see it in his face. He thinks the game is coming to him. You can see it in his face every day,” said Sen. Michael F. Bennet (D-Colo.), a presidential candidate who has argued forcefully for more centrist policies.

Bennet is particularly alarmed by the shift on health care. Among the top four Democrats as measured in national polls, only one — former vice president Joe Biden — has vocally opposed ending private insurance in favor of a government-run plan. Sen. Bernie Sanders (I-Vt.) plans to defend his Medicare-for-all plan in a major speech this week.

“If we nominate someone that is for that plan, we will not win the presidency, and we will have no hope of winning a majority in the Senate,” Bennet said. “We should be on offense on health care. But if we’re going to go into this election talking about taking away [employer-based] health insurance for 180 million people, I guarantee we will be on defense.”

All 10 candidates on the June presidential debate’s second night raised their hands in favor of offering health insurance to undocumented immigrants — even though some later clarified that they didn’t support offering full coverage.

Biden and other more centrist candidates have grown more animated in the aftermath of the debate’s lunge to the left. As if to underscore their concerns, the New York Post put an image on its front page of the candidates raising their hands when asked about health care for undocumented immigrants.

“Who wants to lose the election?” the headline read.

“I wouldn’t be truthful if I said I was not concerned,” said Rep. Jeff Van Drew (D-N.J.), who won in 2018 in a conservative district that Trump carried and was taken aback when he saw hands go up at the debate on another question, whether the candidates favored decriminalizing border crossings.

“That’s another way of saying we’re going to just have open borders. That’s not good for the country,” Van Drew said. “We can’t have endless people coming in. I do believe we need borders, and the border even needs to have physical structure.”

Further underscoring some of those concerns, Sen. Elizabeth Warren (D-Mass.), who has used policy rollouts to climb in the polls, outlined an immigration plan on Thursday that put her firmly on the liberal side of the debate with a call to remake two major immigration enforcement agencies “from top to bottom.”

Two of the most ineffective debate performances came from candidates who could have been a forceful counterweight to the leftward pull. Former congressman Beto O’Rourke (D-Tex.) struggled in a sharp exchange with former housing and urban development secretary Julián Castro, who argued in favor of decriminalizing border crossings. Biden was widely viewed as lackluster — and was knocked off course by Sen. Kamala D. Harris (D-Calif.) in a discussion about busing and segregationists.

The dynamics of the sprawling field for the Democratic nomination have also contributed to the move left, as lesser-known candidates have helped to define the party with provocative statements.

“One of the challenges we have as a party is when you have 25 voices on a good day, it’s hard to communicate an overall platform,” said Celinda Lake, a Democratic strategist and pollster. “Republicans have a huge advantage, not just with the bully pulpit, but they have one voice. We have to make sure all these voices add up to a clear overall Democratic perspective that can be communicated in nine-second sound bites. And that is tough.”

The divisions are a continuation of disputes within the party over what caused its defeat in 2016, and whether the correct course ahead is to appeal to liberal voters, try to win back Democrats and independents who went for Trump, or some merging of the two.

“In my liberal bubble, people are excited about the options we have and the debates on climate and health care and immigration,” said Chris Savage, chairman of the Washtenaw County Democratic Party in Michigan who also runs a liberal news site called Eclectablog. “Democrats haven’t done particularly well being mainstream over the last eight or 10 years. Maybe this new approach will bear more fruit.”

Some of the candidates are comfortable with that direction and say it could benefit the party in the long run. They argue that a government-run health care system would benefit Americans, many of whom suffer financial distress due to medical bills under the private insurance system. They also view decriminalizing border crossings as far more humane than the current system, and they say immigrants who cross the border illegally should face civil, not criminal penalties.

“I think one of the healthy things about the primary process, especially with so many of us in it, is that you are going to see the full range of Democratic opinion reflected, and I think we will come to a balanced and good place,” South Bend, Ind., Mayor Pete Buttigieg said in an interview.

Last year, Democrats were rigorously disciplined with a consistent message, telling voters in race after race around the country that Republicans were set on destroying President Barack Obama’s health-care law and stripping coverage from needy Americans. In television ads and debates, Democrats made the case that Republicans were unfairly demonizing them as a party in favor of open borders. That successful strategy is, at least, threatened.

Earlier in the race, many candidates had embraced abolishing the electoral college, altering the Supreme Court, providing free college tuition and student loan forgiveness, and ending the filibuster — all measures supported by the party’s most liberal voters. But those issues have far less resonance among other voters than health care, the premier issue for years among Democrats, and immigration, the driving force for many Republicans.

Health insurance has emerged as a major fault line among the party, with Warren and Sanders firmly in favor of the Medicare-for-all plan that would significantly limit or eliminate the role of private insurers. Harris has been wobbly on the issue, at times moving toward that proposal but then quickly backing away.

During the debate, she joined Sanders in raising her hand in response to a question asking whether the candidates would abolish private insurance in favor of a government plan. She said the next day that she understood the question to be about her own insurance, not what she would do for all Americans.

“When the debate is in terms of affordability for all Americans and making sure everyone can take care of their family, that’s good ground. That’s how it should be fought,” said Andy Slavitt, who served as acting administrator for the Centers for Medicare and Medicaid Services in the Obama administration and who is now on the board of a health-care-services company. “Where everyone should be uncomfortable is if we have a much more simplistic conversation about some simple-sounding thing without a lot of details that boxes people into a corner.”

Biden hasn’t outlined his full proposal on health care, but he has backed allowing people to keep their employer-based insurance if they prefer. He has said they could also buy into a public option like Medicare — “building on what we’ve got, not starting over,” as he put it.

“I’m going to be very blunt with you: It’s going to be a big debate among us all in this race,” Biden said in Waterloo, Iowa, a week after the debate. “I fundamentally disagree with anyone who says, ‘Scrap Obamacare.’ I’m against any Republican who wants to scrap it, I’m against any Democrat who wants to scrap it.”

In a sign of how widespread anxieties about Medicare-for-all have become, Sanders devoted time in his remarks at a recent campaign office opening in West Des Moines to addressing specific concerns critics have raised about his single-payer proposal. Hinting at some of the political risks of his agenda, he warned that Democrats would have to rebut “lies” about his proposals.

“Despite what anybody may tell you, the Medicare-for-all single-payer program that we are fighting for will give people 100 percent freedom of choice to go to any doctor that they want to go to,” Sanders said.

“People say, well, I’m going to lose my private insurance,” Sanders added. “What Medicare-for-all will do for employers and for workers is give you, finally, stability. It will be there.”

Voters in Iowa in recent days seemed just as divided as the candidates. Ray Frederickson, 56, of Cedar Falls said he sees Biden as a moderating presence in the Democratic field, one who isn’t making promises that he can’t deliver or pay for.

“You can’t just say free college for every kid,” Frederickson said. “What are we going to do about things that affect everyday people? We’re not going to do away with all the health care in this country. We need to shore up the stuff we have.”

But John Deeth, a Democratic blogger and activist in Iowa, said he does not see a political problem with embracing sweeping ideas Republicans appear eager to rebut. Catering to the moderates and centrists poses a bigger risk — disenchanting the party’s core voters in 2020 and dampening turnout in key cities like Philadelphia, Milwaukee and Detroit, he said.

“The Democratic Party,” he said, “can’t throw its base under the bus.”

David Weigel, Laura Hughes, Sean Sullivan and Cleve R. Wootson Jr. contributed to this report.