Bloomberg/Getty Images Preserving and expanding the New Deal Program has become a progressive litmus test, but Hillary Clinton will not rule out any and all Social Security benefit cuts.

Hillary Clinton refuses to rule out any and all benefit cuts to Social Security, angering leading progressive groups that have not endorsed a candidate in the Democratic primary -- and prompting a new challenge from Bernie Sanders.

The issue has arisen as Clinton and Sen. Bernie Sanders (I-Vt.), her remaining rival for the Democratic presidential nomination, debate who has stronger progressive bona fides and progressive groups call for a red-line pledge not to cut benefits.

A Clinton aide instead referred The Huffington Post to the statement on Social Security on the campaign's website, which says the former Secretary of State will "oppose closing the long-term shortfall on the backs of the middle class, whether through benefit cuts or tax increases."

The aide confirmed that the policy position on the site means she's firmly opposed to benefit cuts. “She has no plans to cut benefits and, in fact, has a plan to expand them," the aide said.

But Clinton used stronger language to characterize her opposition to privatizing the Department of Veterans Affairs during Thursday's presidential debate with Sanders.

"Yes, let's fix the VA, but we will never let it be privatized!" Clinton said. "That is a promise."

Sanders has long said he opposes any and all benefit cuts -- and even advanced legislation that would expand the program across the board. The Sanders campaign reiterated that commitment on Wednesday.

Of course. There is no debate about this among progressives. https://t.co/ZDLj8v193z — Bernie Sanders (@BernieSanders) February 3, 2016

The dissatisfaction of progressive groups with the Clinton campaign's statements on Social Security have already provided new fodder for the Sanders campaign as the two candidates enter the home stretch ahead of the New Hampshire primary on Tuesday. Sanders asked Clinton to clarify her position on benefit cuts at a Friday campaign stop in Manchester, New Hampshire.

"I ask Secretary Clinton to join me in making it very clear that number one, she will not support any cuts to Social Security," Sanders said, according to a New York Times reporter who live-tweeted the event. "Number two, she will join me in saying that it is imperative that she increase and expand benefits for senior citizens and disabled."

In November, the Senate passed legislation that eliminated some creative (and dubious) strategies retirees used to boost their benefits; Sanders voted for that legislation, which technically precluded future beneficiaries from using a more lucrative benefits claiming strategy, complicating the dispute.

What worries advocates, however, is that a Democratic president would use Social Security as a chip in a "grand bargain" with Republicans after the election. Clinton pledges on her website to "expand" Social Security benefits for "those who need it most and who are treated unfairly by the current system -- including women who are widows and those who took significant time out of the paid workforce to take care of their children, aging parents, or ailing family members."

Nancy Altman, a co-founder of Social Security Works who has 35 years of experience in the field, said that Clinton campaign's statement and the policy descriptions on her website, do not definitively promise not to cut the program.

"What Secretary Clinton has said about Social Security is completely consistent with the Bowles-Simpson plan," Altman said, referring to a Fiscal Commission proposal in 2010 that would have made major cuts to middle-class benefits, even as it marginally lifted those of poor beneficiaries. "From the very beginning, there have been those who have wanted to boost benefits at the low end and cut middle-class benefits -- pushing it in the direction of becoming a kind of welfare program. It is very important that the candidates not only expand benefits but promise not to cut them. Otherwise there could be cuts that undermine what the program is: insurance, where you get a fair benefit for the money paid."

Altman -- who served as senior policy adviser to the bipartisan Greenspan Commission, which recommended the last round of major Social Security reforms in 1983 -- took issue with the notion that in voting to close loopholes, Sanders had given his imprimatur to cuts.

"People who really understand the program know that those are not benefit cuts," Altman said. "It was a correction of a mistake that was in the law that inadvertently allowed wealthy people and financial advisers to game the system in a way that was never intended."

The Progressive Campaign Change Committee, a digital advocacy group that claims one million members and has yet to endorse a candidate in the presidential race, was more indignant with the Clinton campaign's statement about its stance on Social Security.

It is an absolute must for a Democratic nominee who claims to be progressive to say ... they will never cut Social Security benefits.

"George W. Bush had no plan to invade Iraq," said Stephanie Taylor, a co-founder of the group, referring to the Clinton campaign's assurance that she had no "plans" to cut benefits. "It is an absolute must for a Democratic nominee who claims to be progressive to say clearly and unequivocally that they will never cut Social Security benefits. Bernie Sanders has made that commitment. Hillary Clinton should make that commitment before the New Hampshire primary so Democrats can focus on expanding benefits."

The PCCC claimed that Donald Trump, the Republican presidential frontrunner, has taken a firmer stance against cutting Social Security than Clinton had.

"People have been paying in for years," Trump said in April. "They're gonna cut Social Security. They're gonna cut Medicare. They're gonna cut Medicaid. I'm the one saying that's saying I'm not gonna do that."

The PCCC has lauded Clinton for promoting debt-free college in the past and says its agenda is economic populism, not the prospects of a particular candidate.

Progressive groups began to push Clinton to clarify her position on the program after she nearly tied with Sanders in the Iowa Caucus.

Adam Green, a co-founder of the Progressive Campaign Change Commitee, challenged Clinton to "promise before the New Hampshire primary that she will never cut Social Security," in a Feb. 2 appearance on Fox News this week.

Later that day, Social Security Works, the convening arm of a coalition of labor union, women's and seniors' groups dedicated to expanding Social Security -- and protecting currently scheduled benefits from any cuts -- issued a statement asking for a commitment from Clinton to do the same.

Democracy for America, an online progressive heavyweight that has endorsed Sanders, continued the push for answers from Clinton on Twitter.

Relevant ? that progressive candidates can easily answer: Will you take #SS benefit cuts 100% off-the-table? @BernieSanders @HillaryClinton — DemocracyForAmerica (@DFAaction) February 3, 2016

Protecting Social Security from benefit cuts has become an important test of progressive credibility in recent years, as activist groups and national progressive leaders like Sen. Elizabeth Warren (D-Mass.) cast the popular program as a crucial pillar of social insurance and bulwark against income inequality. Clinton endured criticism for demurring in a previous debate as to whether she would "expand" the program across the board as Sanders, and former Maryland Gov. Martin O'Malley (D), then a Democratic presidential candidate, proposed.

Ryan Grim contributed reporting.

This story has been updated to include Sen. Bernie Sanders' (I-Vt.) call for Clinton to pledge not to cut Social Security benefits at a campaign event on Friday.

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