In a move viewed as further evidence of Sen. Bernie Sanders' continued momentum in the polls and among potential voters in the 2016 presidential election, some Hillary Clinton supporters have gone on the offense against the self-described democratic socialist.

Sen. Claire McCaskill (D-Mo.), a Clinton backer, launched what The Hill described as "the sharpest attack yet" on Sanders in a television appearance on Thursday.

"Bernie is too liberal to gather enough votes in this country to become president," she told MSNBC's "Morning Joe."

McCaskill assailed Sanders for having "an extreme message" and being "unrealistic," while complaining that the media were "not giving the same scrutiny to Bernie Sanders that they are giving to certainly Hillary Clinton and the other candidates."

Asked to name 3 issues she thinks Sanders is 'too far left, too Socialist" on, McCaskill said:

"He would like to see Medicare for all in this country, have everybody have a government insurance policy, I don't think most Americans want government to have every insurance policy... He would like to see expansion of entitlement. He is not worried about a debt at all. He is somebody who is, I think, frankly, against trade. Against a lot of things unrealistic in this day and age."

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Though McCaskill endorsed then-Sen. Barack Obama in 2008, Politico reports that she "endorsed Clinton’s 2016 bid almost exactly two years ago, in June 2013, making her one of the former secretary of state’s earliest major Democratic backers."

The statements came as recent opinion polls have shown support for Sanders building in early battleground states such as New Hampshire and Iowa.

Sanders subsequently struck back in an interview with Bloomberg Politics' Mark Halperin and John Heilemann, calling into question McCaskill's progressive bona fides. "Do I believe, in opposition to Senator McCaskill, that we need trade policies that are fair to the American worker, and not just benefit CEOs and large corporations?" Sanders said. "I plead guilty."

Meanwhile, the Sanders camp is claiming the attacks as a sort of victory.

"When your opponents and people who represent them wade into the conversation with attacks against you, for us it’s a recognition of the fact that something must be working," Tad Devine, a senior adviser to Sanders, told The Hill. "It’s a basic rule in politics that you don’t attack somebody if they are not doing well."