Democratic support for a single-payer health-care system has grown by bounds this year, attracting more lawmaker endorsements than any time in the past. But one group is conspicuously not on board: party leaders.

Senate Minority Leader Charles Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (D-N.Y.) and House Minority Leader Nancy Pelosi (D-Calif.) on Tuesday previewed the much-anticipated release of Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE’s (I-Vt.) “Medicare for all” bill by taking the notable step of refusing to throw their weight behind it.

“Right now I’m protecting the Affordable Care Act,” Pelosi told a group of reporters in the Capitol.

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“Democrats believe that health care is a right for all, and there are many different bills out there,” Schumer echoed a few hours later.

The tepid response highlights the dilemma facing the top Democrats, who are hoping to energize the party faithful without alienating the more conservative-leaning voters that flocked to President Trump last year.

Both Pelosi and Schumer are liberal lawmakers who have supported single payer in the past. But as minority leaders fighting to win back power, they’re also charged with crafting a campaign message that resonates with the centrist voters they’ll need to pick up seats in 2018 and beyond.

Toward that end, Pelosi and Schumer joined forces in July to introduce their 2018 agenda, which focuses on bread-and-butter economic policies supported across a spectrum of ideologies. Single payer, while popular in Democratic circles and gaining steam elsewhere, still polls underwater on a national level.

Sanders, a liberal icon, says those numbers would improve if only the Democrats would embrace the policy more enthusiastically. He championed single payer during last year’s presidential campaign, invigorating the Democrats’ liberal base and distinguishing himself from Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE, the Democratic presidential nominee who declined to endorse "Medicare for all."

On Wednesday, Sanders will introduce his proposal, which has already won support from at least 10 Senate Democrats — a testament to Sanders’s successful White House run and his newfound star power on the national stage. Indeed, a number of those endorsements are from 2020 presidential hopefuls — an indication that ambitious Democrats want to win early favor from the party’s base.

A House bill is also gaining momentum. Sponsored by Rep. John Conyers Jr. (D-Mich.), the measure has the backing of 117 Democrats — more than half of the caucus and by far the most endorsements the bill has enjoyed since Conyers began introducing it more than a decade ago.

Some liberal single-payer advocates are criticizing the top Democrats for declining to get on board despite their past support for the underlying policy.

“You support an ideal, but you’re going to vote against that ideal?” said Zeynab Day, spokeswoman for Brand New Congress. “I think that creates a little bit of an inconsistency.”

If Democratic leaders run the risk of deflating their base by declining to endorse single payer, there are also a number of reasons for their reluctance to do so. First, Pelosi, Schumer and the Democrats spent more than a year ushering the Affordable Care Act (ACA) through Congress — and they’ve spent the past seven years defending it from the torrent of attacks from conservatives.

That debate took a political toll on the party, as the Democrats lost 63 House seats, and the Speaker’s gavel, just months after the law was enacted. Rather than replay that contentious fight, Democratic leaders clearly want to focus on fixing the problems they acknowledge are dogging the ACA.

“There are things in the ACA that aren’t working as well as they should,” Rep. Steny Hoyer (Md.), the Democratic whip, told reporters Tuesday. “The first objective that both leader Pelosi and I have is to preserve the ACA and to adopt policies that will make it work better. That’s our first priority.”

Additionally, the ACA has gained a new popularity amid the Republicans’ failed efforts to repeal it.