By Alex Rogers and Tami Luhby, CNN

(CNN) -- Democratic Rep. Lauren Underwood was a kid in a swimming class when her heart began to race. She was diagnosed with supraventricular tachycardia, which has since shaped her life in every way, from her preference for caffeine-free diet Coke to her career starting out as a registered nurse.

In 2017, when she saw her Illinois congressman, Republican Randy Hultgren, promise to protect those like her with preexisting conditions, but then later vote for a bill that would have increased their costs to get covered, a heart that sometimes beats too fast also gave her a reason to run for Congress.

Democrats took the House in 2018 by campaigning against the Republican efforts to repeal and replace the Affordable Care Act, tying personal stories to a larger narrative of protecting the coverage of millions of Americans. The strategy worked for Underwood, who won her exurban Chicago district in an upset.

"A lot of these provisions impact me personally," she told CNN last year. "I still don't know how to tread water."

Yet while health care propelled the Democrats to success, the issue has already begun to divide the party as its members propose what to do next.

On Wednesday, Rep. Pramila Jayapal, a Washington Democrat and the leader of the Progressive Caucus, and Democratic Rep. Debbie Dingell of Michigan will unveil the most significant single payer proposal in the House yet. It has the support of more than 100 members — many more than support less radical House bills that would permit states to create a Medicaid buy-in option or allow Americans at age 50 to buy into Medicare. It's based off a bill by Sen. Bernie Sanders, who got other senators running for president — Elizabeth Warren, Kamala Harris, Kirsten Gillibrand and Cory Booker — on board too.

Nevertheless it has split the 235-member House Democratic caucus, and the vast majority of the new freshman class — roughly 75% — has not signed onto the Medicare for All bill.

Some of those who haven't co-sponsored the bill are freshmen Democrats from competitive districts like Underwood, Rep. Donna Shalala, the former Health and Human Services Secretary, and Rep. Colin Allred of Texas, who said in interviews that they are focused on fixing the Affordable Care Act and lowering the price of pharmaceuticals rather than a total overhaul of the health care system.

Underwood told CNN that her priority in Congress is to "stabilize the Affordable Care Act and make sure it's affordable for folks seeking coverage," pointing out that there are 37,000 people in her district who use its marketplace.

"I think it's very clear that the American people are looking for lower premium prices, lower prescription drug prices, and higher quality coverage," Underwood said. "There are many routes to achieving those goals."

When it comes to debating one of those routes — such as a government-run plan — she raises a host of questions.

"How much is it going to cost?" she asked. "How are we going to pay for it? Who specifically it covers? What happens with private insurance? What happens to all types of coverage?"

The bill introduced Wednesday aims to answer some of those questions, although not in regards to cost, which would likely run into the trillions of dollars. The lack of those critical details will fuel the doubts of its skeptics.

"We certainly have some great freshmen members who are trying to put out big ideas," Allred told CNN. "And then we have a lot of freshmen members like me who I think are focused on concrete things that we can deliver in this session of Congress — in the next two, four, six years — because we come from districts who expect results."

What's in the bill?

The House bill, named the Medicare For All Act of 2019, would remake the nation's health care system by creating a government-run plan that would insure all Americans. It would provide generous coverage to patients, but could cut payments to many hospitals and doctors.

Like Sanders' bill, the House version would provide comprehensive coverage, including not only primary and specialty care, hospitalization, mental health, maternity care and prescription drugs, but also vision, hearing and dental care. Unlike Sanders' bill, the transition to the new system would take a little more than two years instead of four. Plus, it would provide long-term care services for the elderly and disabled, including covering home health aides, rehabilitation services and nursing home stays.

Insurance premiums, deductibles and co-payments would, for the most part, disappear along with private insurance, including the job-based coverage that now covers half of Americans. Employers and insurers could only sell supplemental coverage for items such as cosmetic surgery. And a fund would be set up to help the workers in the private insurance industry who are displaced to transition to other jobs.

Its supporters talk about Medicare for all in deeply personal, even ethical terms.

Rep. Andy Levin, a Democrat of Michigan, is a two-time cancer survivor with two kids who have Crohn's disease, a chronic inflammatory condition that affects their digestive tracts. In one 2018 campaign ad, he held up an $18,000 bill to treat one of his children. He said in an interview that it's a "moral" issue to him that the country ensures every citizen is insured.

"You might not be here tomorrow," said Levin. "That's what you learn when you get cancer."

"I'm not that much of an incrementalist about anything," he added.

Rep. Alexandria Ocasio-Cortez, the youngest congresswoman ever, co-sponsored the bill since Medicare for all is "part of my values."

"Single-payer is where I think we need to go as a country in order to fully realize the vision of health care as a human right," she told CNN.

Democrats have been pushing national health insurance plans for decades. After Sanders' campaign and the Republicans' attempt to repeal the Affordable Care Act, the idea has gotten modestly more popular in recent years.

About three-quarters of the public support less disruptive changes like a Medicare or Medicaid-buy in plan, while about 56% of respondents favor every American getting insurance from a single government plan,according to Kaiser Family Foundation polling.

"People will like the idea of not paying premiums and deductibles, and not too happy about increased taxes," said Larry Levitt, the senior vice president for health reform at KFF, of Medicare for all.

The fight will last for the foreseeable future. Nobody expects a single-payer bill to be passed by Congress and signed into law while President Donald Trump and Senate Majority Leader Mitch McConnell are in charge.

Unlike passing the Affordable Care Act, which expanded Medicaid to millions and helped millions more buy private health care insurance, Democrats who support single payer will have to not only overcome Republican opposition, but also combat entire industries — hospitals, certain doctors, insurance companies, drug makers, medical device manufacturers — to pass their bill.

'We believe that the market is broken'

On Tuesday, Jayapal said America needs a "complete transformation" of the health care system. She says it is currently "atrocious" and more expensive with worse outcomes than other major industrial countries, calling out the multi-million dollar compensation packages of the Aetna, Cigna and UnitedHealth Group CEOs.

"It is time to put health over profit," she said. "We believe that the market is broken."

But Jayapal's immediate concern is trying to get Democrats on her bill. She told reporters that she was "literally walking" new members who hadn't yet hired health care staffers through it.

"They had a few questions about a few elements of it because they would hear Howard Schultz go on TV and say you're never going to be able to choose your doctor or hospital, which is flatly untrue," she said, referring to the former Starbucks CEO considering an independent bid for president.

She will also have to change the views of her leadership and the chairmen of two of the most influential committees — Ways and Means and Energy and Commerce — who have so far not even committed to holding a hearing on the bill. Asked Tuesday how leaders view the bill, Jayapal said that House Speaker Nancy Pelosi told her they could work together to hold "real discussions" on the issue.

Even House Budget Committee chairman John Yarmuth, a supporter in the past of Medicare for all, is doubtful that the bill will receive a vote this year.

"I'm not sure there would be enough agreement in the (Democratic) caucus on a specific proposal," he said.

Republicans already are predicting that some Democrats who took back the House in 2018 will be vulnerable in future contests, arguing the President's unpopularity overshadowed Democratic campaign platforms.

"The 2018 elections became a referendum on Trump," said Dave Gilliard, a California Republican strategist who saw Orange County flip several seats for the first time in decades. "Issues were secondary."

"I predict many (of) these newly elected Dems will be one- or two-termers," he added. "Once Trump is out of the picture, I expect a return to normalcy."

One of those Democrats who won a Republican seat in California is Rep. Mimi Porter, a supporter of the bill who wants public hearings to debate the topic.

"The onus is on me and on Congress and those of us who support Medicare for all to make our case to the American people," she said.