DES MOINES — Sen. Bernie Sanders (I-Vt.) is seeking to break through a congested Democratic presidential race by campaigning more aggressively on Medicare-for-all, a risky strategy his advisers hope will shift the contest in his favor amid signs he has lost ground in recent months.

On the campaign trail, Sanders increasingly touts his plan for a government program to insure all Americans, reminding voters that rivals followed his lead. And aides and allies have grown more hostile toward competing ideas, while Sanders’s team has planned more frequent events and initiatives focused on health care.

“You talk like ‘universal health care,’ ‘health care for all,’ what does it mean?” Sanders told reporters on a muggy Sunday afternoon at the Iowa State Fair, referencing phrases other White House hopefuls have used. Reprising a line he used in a recent debate, he said, “As some of you may know, I wrote the damn bill. All right? I know a little bit about it.”

But it’s not clear if the emboldened approach will help Sanders or compound his troubles.

Critics have raised concerns about ending private insurance and raising taxes to pay for the proposal. Former vice president Joe Biden, leading in the polls, has railed against it, arguing to expand the Affordable Care Act instead. And voters are showing more uncertainty about Medicare-for-all at Democratic events.

Another challenge, some Democrats say, is that Sanders is no longer the only major candidate supporting Medicare-for-all, potentially making him a victim of his own success. After a breakout campaign against Hillary Clinton in 2016 and a strong start to his 2020 effort, the Vermont independent has struggled to win over new voters. Many of his supporters say they are drawn to other candidates, such as Sen. Elizabeth Warren (D-Mass.), who has moved ahead of Sanders in some recent surveys.

“Is it really a way to differentiate yourself, still?” said Grant Woodard, an Iowa-based Democratic lawyer who worked for Clinton in 2008 and supported her in 2016. “He can sort of claim he’s the original gangster on promoting health care for all, and he can probably try to sell that in a unique way. I just don’t know that that’s going to work.”

Top Sanders officials, however, say health care is a winning issue for the candidate, a powerful tool to connect with people.

“How do you stand out in a crowded field with a lot of candidates running?” said Ben Tulchin, Sanders’s pollster and strategist. Health care, he said, “is an issue where Bernie is seen as the most credible.”

A July Washington Post-ABC News poll found that 27 percent of Democratic-leaning adults said they trusted Sanders most to handle health care, similar to Biden’s 25 percent but higher than Warren’s 13 percent.

His campaign has attempted to capi­tal­ize on this dynamic. In recent weeks, Sanders delivered a speech on Medicare-for-all, took a bus trip to Canada with people with diabetes to purchase insulin at a much lower cost and challenged his competitors to refuse campaign donations from executives and lobbyists of health insurance and pharmaceutical companies.

Sanders, who is generally not a warm politician and tends to spend less time mingling with supporters at events than Biden and Warren, has also tried to put a more personal touch on his health-care pitch. At a Medicare-for-all town hall in Los Angeles last month, participants lined up at microphones to share their thoughts.

During his three-day swing through Iowa, a crucial state that will hold the first nominating contest, he brought up health care most everywhere he went. At a rowdy Democratic Wing Ding fundraiser on Friday, he devoted one of his five minutes onstage to the topic. The next day, he made an impassioned plea for his proposal at event with abortion rights activists. And it was a focal point of his speech at the state fair.

“Will you stand with me to take on the health-care industry and move this country to Medicare-for-all?” thundered Sanders.

The intent, Sanders advisers say, is to seize an opportunity to own health care, a top issue for voters, according to the July Post-ABC poll. It’s also part of a broader pitch the Sanders team is making to working-class voters. They say they believe Sanders offers a more straightforward plan than his competitors — one that voters can easily understand.

Biden supports the expansion of Obamacare with a public health-insurance option. Sen. Kamala D. Harris (D-Calif.), who co-sponsored Sanders’s bill, recently unveiled her own plan, which would take more time to implement and allow private insurance. The Sanders campaign has attacked both plans.

Warren, who has released detailed policy proposals on a number of topics, has notably aligned with Sanders’s plan when it comes to health care. “I’m with Bernie,” she said at the first debate.

Questions about how Medicare-for-all would affect the economy have been surfacing at Democratic events with increasing frequency. At a town hall on Thursday evening, in Fort Dodge, two Iowans asked Warren about implementing single-payer health care; one question focused on supplemental insurance, another on what would happen to the “good people” working in the insurance industry.

“We’ve got a lot of people working in insurance that are not in health insurance,” Warren said. “None of us should deny that as we transition this system over to Medicare-for-all, we’d leave more people in the Medicare system to be able to handle it. Some of those folks who now work on the private side will end up working on the public side.”

The Sanders bill would bar private insurers from replicating services covered by the government plan. But Americans would still have the option to purchase private insurance for elective procedures such as cosmetic surgery, Sanders has said.

Rep. Tim Ryan (D-Ohio), a presidential hopeful who has co-sponsored a House Medicare-for-all bill, attacked Sanders and others who want to implement it strictly as written. In an interview, Ryan emphasized his support for several different health-care proposals, a strategy many prominent Democrats have adopted.

“I’m on that bill; I’m on the bill that doesn’t take away private insurance,” Ryan said. “My position is, how do we get everyone, eventually, at some point, into the Medicare program? But we’re not going to ask anybody to give up private insurance.”

Democrats ran a successful midterm campaign contrasting their efforts to protect Obamacare with Republican attempts to repeal it. Some Democrats worry about losing the high ground in the health-care debate if their standard-bearer in 2020 is also calling for sweeping changes.

The July Post-ABC poll found 52 percent of Americans overall and 77 percent of Democrat-leaning voters prefer adopting a universal health insurance program like Medicare to the current health insurance system. But support dropped to 43 percent among the general public if Medicare-for-all meant there would be no private insurance options available. A 66 percent majority of Democratic-leaning voters still supported it.

A January Kaiser Family Foundation poll illustrated how the public is sensitive to the possibility of tax increases. That survey showed 56 percent supporting a Medicare-for-all health plan, but 60 percent said they would oppose it if it required most Americans to pay more in taxes.

Sanders has acknowledged that his plan would require a tax increase on middle-class Americans. But he has said that it would be more than offset by savings they would achieve from not having to pay health-care premiums and other associated costs.

The differences on health care among the candidates reflect larger divides in the race. While Sanders is rooting his pitch to voters in a sweeping policy, Biden has cast the importance of defeating Trump as the most urgent priority.

“This is about more than any issue we can discuss today,” Biden said in his speech at the Wing Ding.

In his remarks there, Sanders said Trump will not be defeated “unless we have an agenda that speaks to the pain and reality of the working families of this country.”

David Weigel in Des Moines and Scott Clement in Washington contributed to this report.