The rift between the progressive and moderate Democratic presidential candidates is deepening on healthcare.

Sen. Kamala Harris recently ignited another round of controversy on her healthcare position when she distanced herself from the Medicare for All bill that Sen. Bernie Sanders pushed in the Senate — two years after she became its first cosponsor.

In remarks this weekend to wealthy donors at a Hamptons fundraiser, Harris said she has "not been comfortable" with the Sanders proposal, the Daily Beast first reported. Then the California senator told the Washington Post she didn't think her discomfort "was any secret."

Sanders later blasted Harris, and doubled down on his signature healthcare plan.

The Harris comments underscore a new, if subtle, rhetorical shift on the campaign trail among the 2020 Democrats. They've begun tempering their endorsement for the sweeping Sanders plan, acknowledging that eliminating private health insurance remains unpopular with voters, according to recent polling.

Read more: A majority of Americans with employer-based health insurance don't mind if it changes to Medicare for All — as long as they can keep their current coverage

Insider reached out to each of the four Sanders bill co-sponsors running for the presidency: Harris, and Sens. Elizabeth Warren, Cory Booker and Kirsten Gillibrand. The Warren, Harris, and Gillibrand campaigns did not respond to requests for comment asking if they stood by the Sanders bill.

The Booker campaign pointed to answers the New Jersey senator gave The New York Times on a healthcare policy survey back in June. He acknowledged he was a sponsor of the Sanders bill, but said, "the most important thing is to keep the ultimate goal in mind: affordable health care for every American, because health care is a human right."

When pressed for additional specifics, the Booker campaign did not respond to requests for comment.

At the onset of the Democratic primary, endorsing Medicare for All became a litmus test for the candidates, a way of highlighting their progressive credentials in a primary driven by party activists. But voters expressed concern with ending private health insurance, forcing some of the candidates to reevaluate their approach. Strengthening the Affordable Care Act is another idea that's proved popular with Democratic voters.

Read more: 2020 Dems grapple with how to pay for 'Medicare for All'

"What I think has happened in the Democratic primary is people recognize that some of the concerns about single-payer are not coming from special interests but the public," Neera Tanden, president of the Center for American Progress, told the Post.

Like the Green New Deal, the dynamics suggest Democrats view the Sanders Senate bill as the embodiment of a long-term vision, instead of an immediate plan to run on and fully champion.

Harris rolled out her new Medicare for All plan last month, which would allow private insurers to sell health coverage through Medicare under strict regulations, and aim to achieve universal coverage within ten years.

In the past, Booker labeled himself "a pragmatist" when it comes to healthcare reform and has stressed he would "find the immediate things we can do."

Warren still has not come out with her own healthcare plan. Though she said she was "proud" to cosponsor the Sanders bill, she's given herself some breathing room by acknowledging "there are a lot of different pathways" to achieving universal healthcare. Gillibrand recently said at a Washington Post event she liked the idea of a government-run healthcare system that competes with private insurers, otherwise known as the public option.

Among the 2020 Democrats, the battle over the best way to achieve healthcare reform is just getting started.