Sen. Bernie Sanders (I-Vt.) is set to release his long-awaited "Medicare for all" bill Wednesday afternoon with the backing of a number of prominent Democrats.

Sanders has championed single-payer healthcare for decades, but the idea is catching fire among Democrats following his strong run for the White House in 2016.

Several Democrats who could run for the White House in 2020 - a category that includes Sanders - are expected to appear at the bill's unveiling Wednesday afternoon. The bill garnered 16 co-sponsors hours before it was set to be introduced.

Still, not everyone is on board with the Sanders plan. House Minority Leader Nancy Pelosi (D-Calif.), for instance, said Tuesday she is focused on protecting ObamaCare.

Here's where Senate Democrats stand so far:

Supports (16)

Tammy Baldwin (Wis.): Baldwin, who is up for reelection in 2018, will be a co-sponsor of the legislation. "With this reform, we would simplify a complicated system for families and reduce administrative costs for businesses. It would expand coverage to all the uninsured, make health care more affordable for working, middle-class families and reduce growing prescription drug costs for taxpayers," Baldwin wrote in an op-ed in the Milwaukee Journal Sentinel.

Richard Blumenthal (Conn.): On Tuesday afternoon, Blumenthal tweeted: "Proud to announce my support for single-payer #MedicareForAll led by @SenSanders. Let's make healthcare a right, not a luxury."

Cory Booker (N.J.): Booker will also sign on as a co-sponsor, and is another Democrat seen as possible 2020 contender. "This is something that's got to happen. ObamaCare was a first step in advancing this country, but I won't rest until every American has a basic security that comes with having access to affordable health care," Booker told the NJTV News Monday.

Al Franken (Minn.): Franken called Sanders's proposal "a starting point" and "an important marker" toward the goal of universal coverage, while calling for "bipartisan policies that improve our current health care system" in the short term.

Kirsten Gillibrand (N.Y.): Seen as possible 2020 presidential contender, Gillibrand announced Tuesday that she would co-sponsor Sanders's bill. "Health care is a right, not a privilege. This week, I'll proudly join Senator @BernieSanders to co-sponsor Medicare for All," she tweeted.

Kamala Harris (Calif): Harris will also co-sponsor the legislation, she announced in a town hall in late August. "I intend to co-sponsor the 'Medicare for all' bill because it's just the right thing to do," said Harris, another Democrat seen as having White House ambitions. "It's not just about what is morally and ethically right, it also makes sense just from a fiscal standpoint."

Martin Heinrich (N.M.): Heinrich announced Tuesday that he would co-sponsor the bill, saying, "It is time to recognize that health care is a human right and I believe that the best way to make that a reality in our nation is to build on what we all know works."

Mazie Hirono (Hawaii): She announced her support Tuesday, tweeting "I support universal, affordable, accessible and quality health care as a right, not a privilege. A single payer, Medicare for All system is a strong articulation of this principle, which is why I support this bill."

Patrick Leahy (Vt.): Sanders's colleague from Vermont is also a co-sponsor of the legislation.

Ed Markey (Mass.): Markey announced last week that he would cosponsor the bill. On Tuesday, he tweeted: "#Healthcare is a right, not a privilege!"

Jeff Merkley (Ore.): Merkley, the only senator to endorse Sanders during the 2016 primary, will also co-sponsor the legislation, he announced Monday. "Right now, our health care system is incredibly complex, fragmented, and stressful. It would be terrific to have a simple, seamless system where, solely by virtue of living in America, you know that you will get the care you need."

Brian Schatz (Hawaii): Schatz is also a cosponsor of the bill.

Jeanne Shaheen (N.H.): Shaheen announced her support on Wednesday, tweeting: "I've signed-on to the Medicare For All Act introduced by @SenSanders."

Tom Udall (N.M.): In a statement released Tuesday, Udall announced he would co-sponsor the legislation. "I believe that health care is a human right, and that all New Mexicans - and all Americans - should be able to see a doctor when they're sick."

Elizabeth Warren (Mass.): Also considered a potential 2020 presidential candidate, Warren said last week that she would co-sponsor the legislation. "I believe it's time to take a step back and ask: what is the best way to deliver high quality, low cost health care to all Americans?" Warren said in a statement.

Sheldon Whitehouse (R.I.): Whitehouse said in a statement this week that he would co-sponsor the legislation. "We have come a long way under Obamacare, but I still hear from Rhode Island families and small business owners that health care costs are too high. I am committed to bringing down those costs while improving the quality of care for Rhode Islanders," he said.

Undecided (5)

Tom Carper (Del.): The Delaware senator said the single-payer idea deserves a hearing. "Right now we're trying to focus on the main thing. And ... the main thing is to stabilize the exchanges. Do that now. The idea of 'Medicare for all,' the proposal from Lindsey Graham and Bill Cassidy, these are ideas, big ideas ... those are the kinds of ideas that should have a full-blown hearing."

Bob Casey Jr. (Pa.): When asked his thoughts on the bill, Casey said he hasn't yet seen it and would like hearings on the proposal. He said a "giant step forward" in increasing coverage - that could be implemented now - would be adding a public option to compete alongside private plans. Casey is up for reelection next year.

Joe Manchin (W.Va.): Manchin told The Hill he thinks the idea of single-payer care should be explored, but wouldn't specifically commit to the Sanders bill. "Let it go through the committee, let it go through the process. I don't just know enough about it. I'm not signing on to a piece of legislation that I don't have any idea what it's going to do to the economy, to the access and to people's care."

Charles Schumer (N.Y.): The Senate minority leader says, "there are are many different bills out there. There are many good ones." At his biweekly press conference on Tuesday, Schumer declined to say if he would be signing onto Sanders's proposal. He noted that other Democratic senators have put forth health care bills: "We want to move the issue forward. We're looking at all of these."

Chris Van Hollen (Md.): He said he needs to examine the legislation once it's introduced. "I support making sure that we have a system of universal, affordable health care. There's going to be other proposals introduced. I've always supported moving in the direction of 'Medicare for all,' I've just got to look at the details," he told reporters Tuesday.

Not endorsing (4)

Tim Kaine (Va.): Hillary Clinton's former running mate said he would "rather open it up to more choices, not fewer." Kaine told reporters that he favors adding a public option to the mix of health care insurance policies Americans can choose. He offered support for the introduction of the bill, saying "Look, we have to have all the ideas on the table, so we can flesh them out."

Claire McCaskill (Mo.): McCaskill, who is considered vulnerable in next year's election, told reporters, "I think that particular proposal is premature." Additionally, she said she supports letting people between the ages of 55 and 65 buy into Medicare and other public option ideas.

Debbie Stabenow (Mich.): Stabenow, who is up for reelection in 2018, told reporters she supports letting those 55 and older buy into Medicare.

Jon Tester (Mont.): Tester told reporters he won't be supporting Sanders's legislation. "I support fixing what we got because I think that's more likely to happen." Tester is up for reelection in 2018 in what's likely to be a tight race.

- Sylvan Lane contributed.

Updated: Sept. 13 at 12:20 p.m.