The idea, which had been kicking around liberal circles for decades, was seen as a nonstarter for any serious candidate because of the expected massive price tag of the government paying for health coverage for everyone. And the fact that such a massive cost would almost certainly be paid for by significant tax hikes.

Hillary Clinton suggested the idea was a pipe dream that could undermine her work to shore up the Affordable Care Act. "People who have health emergencies can't wait for us to have a theoretical debate about some better idea that will never, ever come to pass," she said in January 2016

Boy, have things changed!

On Tuesday, New York Sen. Kirsten Gillibrand became the fifth Democratic presidential aspirant to announce support for Sanders' "Medicare For All" legislation. Gillibrand joins Sens. Kamala Harris (California), Cory Booker (New Jersey), Jeff Merkley (Oregon) and Elizabeth Warren (Massachusetts) as co-sponsors for Sanders' legislation. ( Sanders will formally unveil the legislation today .)

What that means is that with the notable exception of former Vice President Joe Biden, every top tier(ish) 2020 Democrat is now on board with a policy proposal that Clinton said less than two years ago would "never, ever come to pass."

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"This is a pivotal moment in American history. Do we, as a nation, join the rest of the industrialized world and guarantee comprehensive health care to every person as a human right? Or do we maintain a system that is enormously expensive, wasteful and bureaucratic, and is designed to maximize profits for big insurance companies, the pharmaceutical industry, Wall Street and medical equipment suppliers?"

To be clear: Sanders' single payer plan has zero chance of passing through the Republican-controlled Senate. No GOP senator will vote for it and it's not at all clear that many of the 10 Democrats up in 2018 in states that President Donald Trump won in 2016 will either.

But, the number of 2020 wannabes willing to put their names to single payer still is important. Why? Because it shows that the energy in the Democratic Party is entirely within the liberal base. And for that base, it's not possible for a candidate to be "too" liberal on, well, anything.

It also means the party is moving toward what was once considered the tilting-at-windmills approach of Sanders (who, it's worth noting, isn't technically a Democrat) and away from the reasoned pragmatism that Clinton preached in the 2016 campaign.

That movement comes even as Clinton hits the media circuit to promote a book detailing her account of the 2016 election -- an account filled with blame for Sanders who, she said, attacked her in ways that left her badly wounded for the general election.

While it's clear the Democratic activist base wants the most liberal possible candidate to take on Donald Trump in 2020, it's less clear whether the broader public will go for a candidate advocating for single payer, or other long-time wish list proposals of liberals.

The sheer number of serious Democrats siding with Sanders suggests we may get an answer to that question come 2020.