In her quest to secure the Democratic presidential nomination, Hillary Clinton has alternately described herself as "a progressive who likes to get things done" and "kind of moderate and center." So Sen. Bernie Sanders (I-Vt.) touched a nerve this week when he said in New Hampshire that Clinton is only a progressive on "some days."

"Some days, yes," Sanders told a reporter Tuesday. "Except when she announces that she is a proud moderate. And then I guess she is not a progressive."

"I think, frankly, it is very hard to be a real progressive and to take on the establishment in a way I think has to be taken," he went on, "when you become as dependent, as she has, through her super PAC and in other ways, on Wall Street or drug company money."

Clinton responded to Sanders Wednesday by saying it was "a low blow" for him to cast doubt on her progressive credentials. She listed various policy efforts she's undertaken, like helping to create the Children's Health Insurance Program and defending Social Security from privatization in the Senate.

"So let's keep it on the issues, because if it's about our records, hey, I'm going to win by a landslide on Tuesday," Clinton said. (The New Hampshire primary is on Tuesday, where Sanders has consistently carried a large lead in the polls.)

Supporters of Clinton came to her defense. Progressive Sen. Barbara Boxer (D-Calif.), for instance, tweeted that "Hillary is a progressive EVERY day. Bernie is a Democrat 'some days.'"

The spat continued on Twitter, with Sanders tweeting that Clinton couldn't claim to be both "a moderate and a progressive" at the same time. He cited some of Clinton's past policy positions to make his point.

Most progressives I know are firm from day 1 in opposition to the Trans-Pacific Partnership. They didn't have to think about it a whole lot. — Bernie Sanders (@BernieSanders) February 3, 2016

Most progressives I know were against the war in Iraq. One of the worst foreign policy blunders in the history of the United States. — Bernie Sanders (@BernieSanders) February 3, 2016

Clinton replied by suggesting Sanders had plunged the two into an unnecessary debate over semantics, while also calling out the senator for his past votes on gun safety.

2) Now, if you do want to make it about who's a "real progressive," @BernieSanders, what were you on these days? pic.twitter.com/8Q6hANYPhh — Hillary Clinton (@HillaryClinton) February 3, 2016

4) Hillary's not running to make a point—she's running to make a difference. She'll keep doing that. Please feel free to keep tweeting. — Hillary Clinton (@HillaryClinton) February 3, 2016

The fight over political labels isn't a new one. In June, Sen. Claire McCaskill (D-Mo.), a vocal supporter of Clinton, complained that she "very rarely read in any coverage of Bernie that he's a socialist" and suggested that he's "too liberal" and "extreme" to win the nomination. That Sanders is a self-described Democratic socialist didn't pose a problem for him in the Iowa caucus, where he narrowly lost to Clinton Monday, since 43 percent of Democratic caucus-goers in Iowa identify as socialists.

And Clinton has not always been seen as a steadfast liberal even by people close to her. In 2005, for example, Bruce Reed, a longtime aide to former President Bill Clinton, characterized Hillary -- at the time a Democratic senator for New York -- as "quite culturally conservative," and therefore "a natural choice" to define a policy agenda for the Democratic Leadership Council, a centrist group with which she had a leadership role.

Reed, the president of the DLC and and Bill Clinton's former senior policy adviser -- who wrote the phrase "end welfare as we know it" into one of the former president's speeches -- pushed back on the suggestion that Hillary had "a reputation for being on the liberal end of the party" in a 2005 interview with NPR.

"Well, I wouldn't say that," Reed said. "I think -- you know, I've known her for 15 years and she's a Clinton Democrat from the get-go. She's always been quite culturally conservative."

"I worked with her on welfare reform," he went on. "She did a number of things in the '90s that didn't get that much attention, like making it easier for adoption and reforming the child welfare system. She's always been a budget hawk. So despite the caricature that conservatives would like to stick on her, I think she is a natural for this."

It's this history that makes some of Sanders' progressive supporters question whether Clinton is really one of their own. Clinton, for her part, has worked to distance herself from the welfare reform, crime and financial deregulation bills of the 1990s that present-day progressives abhor.

Asked Wednesday whether he thinks his 2005 characterization of Clinton is still valid, Reed told The Huffington Post that, to him, Clinton is "exactly what she says she is -- a progressive who gets things done."

"I think that she has spent the last 40 years at the cutting edge of progressive causes and has a lot of progress to show for it," Reed said. "The results matter more than the labels, but by any measure, she's one of the great progressive leaders of our time."

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