Sen. Elizabeth Warren (D-Mass.) says it’s time for Democrats to run on single-payer health care across the country.

President Barack Obama “tried to move us forward with health care coverage by using a conservative model that came from one of the conservative think tanks that had been advanced by a Republican governor in Massachusetts,” Warren, referring to Mitt Romney, said in an interview with The Wall Street Journal published Tuesday.

“Now it’s time for the next step. And the next step is single payer,” she said.

Warren’s comments represent a shift to her position on the U.S. health care system. In March, she said her support for switching to single-payer ― in which the government handles coverage of health care costs, rather than insurance companies ― would depend on whether Democrats could find Republican lawmakers willing to help fix the Affordable Care Act passed under Obama.

Republicans, however, have focused on trying to repeal and replace Obamacare. And neither a bill the House passed nor one the Senate is considering ― both of which would cause more than 20 million people to lose health insurance, according to Congressional Budget Office estimates ― has any support among Democrats.

Last week, a high-profile effort to establish a single-payer health care system in California stalled amid concerns from both Democratic and Republican lawmakers in the state over how to pay for the estimated $330 billion to $400 billion measure. California Assembly Speaker Anthony Rendon (D) called the measure “woefully inadequate.”

National Democrats have been reluctant to call for putting single-payer at the top of their party platform. House Minority Leader Nancy Pelosi (D-Calif.), who supports single-payer, rejected the idea earlier this year, saying the issue should be left up to the states because Congress is not ready for it yet.

Republicans say single-payer is unpopular and turns off voters. Matt Gorman, communications director at the National Republican Campaign Committee, goaded Democrats to heed Warren’s call for such a system.

The White House is also using the specter of single-payer to persuade Senate Republicans to vote for their party’s health care bill.

White House Press Secretary Sean Spicer tweeted that plans by Sen. Bernie Sanders (I-Vt.) to introduce a bill that would create a single-payer system “lays out” the choice for GOP senators wary of the bill their leaders unveiled last week.

In her Journal interview, Warren called on Democrats to ditch half-measures and commit to progressive policies that they believe in.

“It’s not like we’re trying to sell stuff that people don’t want. … It’s not that at all,” she said. “It’s that we haven’t gotten up there and been as clear about our values as we should be, or as clear and concrete about how we’re going to get there.”

Senate Democrats who spoke with HuffPost on Tuesday were cool to the idea of endorsing national single-payer at a time when the party is engaged in a heated battle over the Republican health care bill. Senate Majority Leader Mitch McConnell (R-Ky.) postponed a vote on the legislation on Tuesday, but he still intends to bring a retooled bill to the floor after the July 4 recess.

“If we succeed in stopping this bill, then we have to have a conversation about what’s the best alternative,” Sen. Chris Coons (D-Del.) said on Capitol Hill Tuesday. “But I think we should stay focused on helping our constituents understand what is wrong with [the GOP bill].”

Sen. Sherrod Brown (D-Ohio), another high-profile progressive in Congress, echoed the sentiment.

“My mission is to do all I can to defeat ACA repeal,” he said. “I don’t want to start putting alternatives out there. Right now I want to focus on defeating repeal. There’s 20 million people’s lives at stake.”

This story has been updated to include comments from Coons and Brown.