(CNN) A lack of agreement and a crush of urgency make health care coverage the most important issue in the Democratic primary .

Disagreements among the candidates have largely been focused at the three primary debates on how to go about giving every American health insurance and how to pay for it.

On other issues, there are differences around the edges and some have more and grander plans than others. But they all largely agree that something drastic and immediate needs to be done about climate change. They all want to take on the National Rifle Association. They all -- very, very much -- want to defeat President Donald Trump.

But they are completely split on what to do about health insurance. And that just happens to be among the top issues for voters at large and most especially Democratic voters.

In a CNN poll conducted by SSRS in September , 89% of Democratic and Democratic-leaning registered voters said health care was either extremely or very important. Smaller, though still large, percentages said the same of climate change, gun policy and the economy.

For Republicans and Republican-leaning registered voters, health care was just the fourth most-cited issue -- 73% of them said health care was either extremely or very important to their votes for president. Fewer Republicans cited health care than cited the economy, immigration and gun policy.

But while Democrats -- candidates and voters -- almost universally agree that health care is an important issue, they seem hopelessly split on what to do about it.

In a Pew survey in July , 53% of Americans said the government has a responsibility to make sure Americans have health care coverage, compared with 44% who said the government does not.

But there is a massive partisan split. Just 19% of Republicans and Republican-leaning adults in the poll said the government has a responsibility, compared with 81% of Democrats and Democratic-leaning adults.

And that's just where the disagreements get started.

While most Republicans don't want the government involved in health insurance, 64% of them say Medicare should be continued.

The largest bloc of Democrats and Democratic-leaners, 44%, say there should be a single national government health insurance program (ala Bernie Sanders and Elizabeth Warren and "Medicare for All"). But a large and important 34% of Democrats say there should be a mix of government and private programs (ala Joe Biden, Amy Klobuchar, Pete Buttigieg and Beto O'Rourke).

Add to all of this the party's history with the issue.

When President Barack Obama swept into office in 2009 with a filibuster-proof Senate majority, and after spending a fair amount of political capital to help an economy reeling from the Great Recession, he and Democrats spent all they had left passing the Affordable Care Act.

Without any help from Republicans, they cobbled together votes among Democrats for what became known as Obamacare. Despite no end of trying, they were unable to add a public health insurance option for people not eligible for Medicare. They had to bend Senate rules to get the thing passed. And then they lost the House of Representatives and have spent every year since trying to protect the law from Republican assault. That means what they passed in 2010, though flawed, has been frozen in time.

Sanders, by the way, was among the liberals in 2010 who supported the Affordable Care Act only grudgingly to get something, anything, done back then.

It's led to a 2020 primary in which every candidate wants more government involvement in the health insurance market but there are growing and heated disagreements over how.

The candidates are all deferential to the legacy of Obama while arguing that his signature achievement is either hopelessly flawed, completely inadequate and must be replaced as soon as possible with a government plan for everyone -- or at the very least in need of a major overhaul.

Here, in four quotes from a transcript of Thursday's Democratic presidential debate, is the difference in the party, half of which wants to start over with Medicare for All and half of which wants to improve the Affordable Care Act.

Former Vice President Joe Biden, who wants to make private insurance more affordable

I know that the senator (Warren) says she's for Bernie. Well, I'm for Barack. I think the Obamacare worked. I think the way we add to it, replace everything that has been cut, add a public option, guarantee that everyone will be able to have affordable insurance, number one.

Sen. Elizabeth Warren of Massachusetts, who wants to get rid of private insurance

I've actually never met anybody who likes their health insurance company. I've met people who like their doctors. I've met people who like their nurses. I've met people who like their pharmacists. I've met people who like their physical therapists. What they want is access to health care. And we just need to be clear about what Medicare for All is all about. Instead of paying premiums into insurance companies and then having insurance companies build their profits by saying no to coverage, we're going to do this by saying, everyone is covered by Medicare for All, every health care provider is covered.

Sen. Bernie Sanders of Vermont, who wants to bring the US in line with other countries

Americans don't want to pay twice as much as other countries. And they guarantee health care to all people. Under my Medicare for All proposal, when you don't pay out of pocket and you don't pay premiums, maybe you've run into people who love their premiums, I haven't.

South Bend, Indiana, Mayor Pete Buttigieg, who doesn't want to force Americans into any particular health plan

The problem, Sen. Sanders, with that damn bill that you wrote, and that Sen. Warren backs, is that it doesn't trust the American people. I trust you to choose what makes the most sense for you. Not my way or the highway.

And this doesn't even get to the issue of how to pay for these plans, which is another key difference. Biden and others argue that taxes will go up. Sanders and Warren argue that premiums will go down, canceling that out. The math all gets very fuzzy.

But, and this might be more of a general election discussion, but neither a public option nor Medicare for All seems very likely to pass through a Republican-controlled Senate. It won't even be a discussion if Trump is reelected. So there's a good chance that the entire health care disagreement among Democrats might end up being academic.