CORALVILLE, Ia. — Straining to speak over the lively happy hour crowd at Backpocket Brewing, former Colorado Gov. John Hickenlooper acknowledged he receives “a lot of heat” on the campaign trail when it comes to health care.

Hickenlooper is among a handful of presidential candidates opposed to the growing chorus of Democrats pushing for “Medicare for All,” a plan to expand the national health care program for seniors to all Americans. The plan would replace virtually all private insurance plans with government coverage.

Just days after several high-profile candidates co-sponsored a Sen. Bernie Sanders-backed Medicare for All bill in the U.S. Senate, Hickenlooper cautioned against making that plan a deal breaker for Democratic voters looking to win back the White House in 2020.

"We’re not going to beat Donald Trump if we hold ourselves to a litmus test of ideological purity," he said between sips of his 6th Anniversary Stout. "We have to be pragmatic."

The number of candidates embracing Medicare for All — an idea that dates back to President Teddy Roosevelt — reveals how much the Democratic Party has shifted on the issue in recent years. In 2016, Sanders was the lone supporter. Now, more than half the field supports the idea or some version of it.

But holdouts remain.

The issue has become one of the sharpest dividing lines in history’s largest and most diverse field of Democratic presidential contenders.

Iowa voters are paying attention. In a March Iowa Poll sponsored by the Des Moines Register, CNN and Mediacom, 81 percent of likely Democratic caucusgoers say they hope candidates will spend “a lot” of time talking about health care, and 84 percent say they prefer a candidate who supports Medicare for All, either all at once or incrementally.

On the trail, Hickenlooper frequently says basic health care is a right.

He believes a public option allowing Americans to choose to buy into Medicaid or Medicare would be a practical first step toward universal coverage.

"But let that be an evolution, rather than a revolution," he said. "Because in this country, generally we don’t like government coming in and forcing us."

Klobuchar pushes 'a different way of getting there'

For years, Sanders has pushed for expanding the national health care program for seniors, snuffing out a system of premiums, deductibles and copays with taxpayer-funded care for all.

Last week, he introduced the Medicare for All Act of 2019. Sens. Cory Booker of New Jersey, Kirsten Gillibrand of New York, Kamala Harris of California, and Elizabeth Warren of Massachusetts were among the 14 who co-sponsored the bill.

Minnesota Sen. Amy Klobuchar is the only senator running for president without her name on the 100-page bill — underscoring her belief that Medicare for All is not realistic.

"It could be a possibility in the future," she said at a CNN town hall in February. "I'm just looking at something that will work now."

Like Hickenlooper, Klobuchar wants the U.S. to attain universal insurance coverage, in part by implementing a public option to buy into either Medicare or Medicaid. She’s previously co-sponsored bills to expand both systems.

Under her plan, private insurers would still be able to compete.

Klobuchar said her end goal is similar to that of her fellow Democratic senators.

"I have just a different way of getting there," Klobuchar told the Des Moines Register Friday in Nevada, Iowa, where she toured an ethanol plant.

Even Medicare for All backers have their doubts

Some presidential candidates signed onto Sanders' bill have wavered on the issue.

In January, Harris elicited backlash from Republicans and moderate Democrats on social media when she said she was ready to end private insurance as a means to achieve Medicare for All during a CNN town hall. The next day, one of her staffers told the cable network that Harris is open to alternative routes to a single-payer system.

Ultimately, her staff said, Harris hasn’t changed her position.

Put differently by another aide on Twitter: If Harris wants a burrito but would accept tacos in the meantime, she still wants a burrito.

Likewise, Booker has been criticized for saying he wouldn't do away with private health care. Despite his support of a nationalized system, he recently said: "Even countries that have vast access to publicly offered health care still have private health care."

Former Texas Congressman Beto O'Rourke addressed the issue on his first campaign trip to Iowa, saying his goal is to "get to guaranteed high quality, universal health care for all."

But O'Rourke told reporters in Washington, Iowa, last month that he is "no longer sure (Medicare for All is) the fastest way for us to get there."

John Delaney, a former Maryland congressman, has a more rigid stance: He says Americans don't want a health insurance system solely administered by the government.

"Eliminating private health insurance will decrease access and quality in health care and doom any chance of creating a universal health care system, yet it remains the type of talking point that may sound good but is bad policy," he said in a statement following the introduction of Sanders' bill.

'This has become a pretty tough issue for Democrats'

The Progressive Policy Institute (PPI), a Washington think tank that describes itself as "radically pragmatic" recently traveled to Iowa to encourage an incremental health care approach. It held a panel discussion at Des Moines' Principal Park pushing for alternatives to nationalized health care.

"This has become a pretty tough issue for Democrats," said Will Marshall, president and founder of PPI. "A lot of people are for it at a very high level of generality. When you probe little deeper, sometimes some of that support melts away."

He pointed to a 2016 referendum in Colorado, which sought to roll out a single-payer health care plan across the state, as the last time the issue was subject to the ballot box.

"It got crushed," Hickenlooper answered. Nearly 80% of Coloradans voted down the idea in 2016, when Hillary Clinton won the state by about 5 percentage points over Trump. But Hickenlooper notes that he expanded health care access in his state. Under his watch, Colorado delivered health insurance to 600,000 more people.

In Coralville, in liberal Johnson County, the former governor's hesitancy on Medicare for All earned him a few groans.

Megan Schwalm, a 39-year-old diversity consultant, described Hickenlooper's "evolution-over-revolution" line as a "cringe moment." The Johnson County native now lives in Nashville, but frequently returns to Iowa to see presidential candidates.

"On paper that sounds great," she said. "But there are so many people in our country who can't wait for that evolution. We need a revolution."

But later at a small house party in West Liberty, a group of mostly current and retired teachers nodded their heads in approval as the governor laid out his vision.

"I thought he had a realistic view," said Becky Furlong, a retired school administrator and university instructor from nearby Letts. "It would be wonderful if we could jump to Medicare for All, but I also agree when he says that some of our current health insurance plans work very well for some people."