Warren, a front-runner in the 2020 presidential primary, is known today as one of the most vocal champions for Medicare for All in the Democratic Party. But during her first Senate campaign in 2012, she and her aides were pressed on single-payer and dodged the question again and again, according to an analysis of newspaper reports, TV, and interviews with health care reform activists at the time.

“It was frustrating,” Ture Turnbull, former executive director of Mass-Care, a single-payer advocacy group in Warren’s home state of Massachusetts, said of her comments in the 2012 race. "There were multiple times where we would approach her, and that was the standard response.”

Warren's refusal to embrace single-payer during that campaign came four years after she co-wrote an essay that called it the "most obvious" solution to the nation's health care woes — though perhaps "politically unacceptable."

Warren’s remarks in 2012, as well as her more circumspect attitude toward Medicare for All at the beginning of her presidential campaign, has exposed her to criticisms from the left. That could become a liability, particularly if her progressive rival Bernie Sanders contrasts it with his decades-long support for single-payer. The two candidates have agreed to a non-aggression pact, but as Warren has risen in the polls, Sanders and his aides have begun to highlight differences between the candidates.

In an interview with POLITICO, Sanders said no other presidential contender would prioritize single-payer as much as him. “I'm glad that we have other people, in various forms, supporting it. I've been on this issue for I don't know how many years — 20, 30 years, okay?” he said. “It’s been my passion. I believe from the bottom of my heart that healthcare is a human right.”

Warren’s record on Medicare for All has drawn scrutiny from some single-payer advocates. Many activists trust that she is fully on board with the policy and would push for it in the White House as strongly as Sanders, according to a survey of nearly 20 people and organizations that support Medicare for All. But some, including even Warren’s allies, are less certain.

“We know that if Elizabeth Warren is elected president, we’re going to have to work hard to make sure she prioritizes it appropriately," said Sal Rosselli, president of the National Union of Healthcare Workers, which has endorsed both Sanders and Warren. "We know that Bernie Sanders will because of history."

Warren’s embrace of Medicare for All in 2020 has proven pivotal. It has won her admiration from progressives who see it as a litmus test, and fueled attacks from moderates who say it makes her “unelectable.” It also appears to have helped her chip away at Sanders’ base: A recent poll of the first-in-the-nation primary state of New Hampshire found that more voters who approve of single-payer back Warren than Sanders.

Warren's previous hesitance toward Medicare for All could give Sanders' supporters, if not Sanders himself, an opening to attack her as less than authentic. Some of his fans already have gone there on social media. Health care is also a top concern for Democratic voters, and single-payer is fairly popular within the party, particularly among the progressives who are a key part of Warren's coalition.

“Elizabeth believes Medicare for All should be the law of the land. She has made the case for why consistently and passionately,” said Saloni Sharma, Warren’s national deputy press secretary. “She spent her whole career studying why families go broke, and came to find that one of the top reasons for personal bankruptcies is health crises and medical expenses — even for families with insurance.”

Warren’s aides did not provide a comment on her remarks during her 2012 campaign. It was a vastly different time politically: The “public option,” now seen as the compromise position of Joe Biden, was considered far-left. Fierce backlash against the Affordable Care Act had helped Republicans pick up 63 House seats two years earlier. Brown had won office promising to oppose the bill.

In 2012, Brown tried to paint Warren as a radical who’d usher in European-style, government-run health care. Warren also had a primary opponent, Marisa DeFranco, who called herself the "only candidate in this race who supports single-payer" and chastised Warren for not doing so.

Warren and her staff repeatedly tried to steer questions about single-payer back to the Affordable Care Act.

“I think the urgent question now is whether we're going to be able to hold on to the health care reforms that just passed," Warren said in one typical statement that year. "There are a lot of people who want to repeal them. I think we need to focus on protecting them and on finding new ways to lower costs, which are still too high.”

In another example, Warren's 2012 spokeswoman said"Elizabeth supports the [ACA]" when asked for her position on single-payer.

Braude, the TV host, tried but failed to push Warren off her talking points. “You do support single-payer, do you not?” he asked. Warren replied, “No, what I’ve got right now … ” at which point he interrupted her and countered that she had expressed support for single-payer in the past. She shot back, “Oh! I think you need to go back and take a look.”

Asked this March by CNN's Jake Tapper whether she backed eliminating commercial insurance, Warren said Sanders' Medicare for All bill, which she signed onto in 2017, has "a runway for that," adding later, "I've also co-sponsored other bills." More recently she's come under fire for referring to Medicare for All as a "framework," though she reiterated her support for the plan in the same remarks.

Dying activist Ady Barkan, one of the nation’s most high-profile advocates for Medicare for All, interviewed Warren about single-payer in a video released last month. In the Q&A, he said health care reformers had criticized her at the beginning of her campaign for not talking about Medicare for All or being clear about her stance on abolishing private insurance.

Barkan asked Warren in the interview if she was initially hesitant at the outset because, in his view, her approach to dealing with problems generally is "not to dramatically expand the size of the public sector, but to instead heavily regulate private-sector actors and beat down their greed.” Warren said "no," adding, “I think it was more about focused on transition than on endpoint. But there are areas where markets just don't work, and a big part of health care is one of those.”

Barkan told POLITICO that Sanders deserves "tremendous" credit for rallying for Medicare for All when it wasn’t popular, but aside from that, “there’s really no daylight” between the two candidates on the topic — and they would both make it a top priority in the White House.

“The country thought Medicare for All was a fringe idea back then,” he said of Warren’s comments in 2012. “It was an entirely different context from now, when we have a majority of Americans getting behind single-payer health care.”

Single-payer advocates appear split on how significant Warren's earlier reticence is.

“I know either of them, if elected president, would work tirelessly to grow public and legislative support for Medicare For All,” said Rep. Pramila Jayapal (D-Wash.), the leading sponsor of the House’s single-payer bill who has not yet endorsed a candidate.

But others predicted Sanders would make it more of a focus than Warren or that there are other distinctions between the candidates on the issue. Single-payer supporters who have endorsed Sanders, in particular, are skeptical.

“She’s an Elizabeth-come-lately,” said Peter Knowlton, general president of the United Electrical, Radio and Machine Workers of America, which is backing the Vermont senator.

It's unclear if Sanders will draw stronger contrasts with Warren on Medicare for All. For months he went out of his way not to criticize Warren, whom he considers a friend, but last week he went further than ever in explaining how he differs from her.

"Bernie wrote the bill," said Megan Svoboda, a member of the Democratic Socialists of America's national political committee. "It’s been his priority for years, and he ran on Medicare For All in 2016. There’s no question that it will be a Day One priority for him. Warren, on the other hand, hasn’t prioritized it in her campaign and therefore likely wouldn’t as president."