Sen. Claire McCaskillClaire Conner McCaskillHillicon Valley: Verizon, AT&T call off data partnerships after pressure | Tech speaks out against Trump family separation policy | T-Mobile, Sprint make case for B merger Senators introduce bipartisan bill to detect supply chain risks posing threats to national security Manchin becomes final Democrat to back bill preventing separation of immigrant families MORE is accusing the media of treating Sen. Bernie SandersBernard (Bernie) SandersHeckler yells ‘Mr. President, f--- you’ as Trump arrives at Capitol Veteran New York Dems face upstart challengers Senate passes 6B defense bill MORE with kid gloves during the first few months of his presidential campaign to bolster the narrative that he’s mounting a strong challenge to Democratic front-runner Hillary ClintonHillary Diane Rodham ClintonLewandowski says 'womp womp' at story of young girl being separated from mother at border Giuliani: FBI asked me about tease of a 'surprise' before election Republicans tear into IG finding on Clinton probe MORE.

“The media is giving Bernie a pass right now, I very rarely read in any coverage of Bernie that he is a socialist,” she said Thursday on MSNBC’s “Morning Joe,” referring to the fact that Sanders (I-Vt.) regularly identifies as a Democratic socialist.

“I think everybody wants a fight and they are not giving the same scrutiny to Bernie Sanders that they are giving to certainly Hillary Clinton and the other candidates.

McCaskill (D-Mo.), one of Clinton’s first supporters ahead of her 2016 bid, added that Sanders’s pitfalls will become “very clear” once the media starts treating him like a “serious candidate.” She added that journalists underestimate Clinton’s wide leads, which have her about 50 percentage points ahead of Sanders in national polls, and are putting too much stock into strong attendance at his political rallies.

“Any other candidate that had the numbers that Hillary Clinton had right now would be talked about as absolutely untouchable, but all of a sudden, ‘Bernie, Bernie, Bernie,’ ” she said.

“It’s not unusual for someone who has an extreme message to have a following.”

Sanders is in the midst of an attempt to outflank Clinton on the left with a largely populist economic platform. He’s drawn within 12 percentage points in some polls in New Hampshire and continues to gain steam, but he’s still well behind Clinton.

McCaskill believes Sanders’s liberal message puts him too far away from the center to win a general election, specifically pointing out his views on expanding entitlements and against President Obama’s signature trade deal.

Sanders embraced most of those characterizations in a Thursday interview, but pushed back against the idea that the media doesn’t regularly refer to him as a Democratic socialist.

“Do I believe in a Medicare for all single-payer system? Absolutely. Do I believe in opposition to Sen. McCaskill that we need trade policies that are fair to the American worker and not just benefit CEOs of large corporations? I plead guilty,” he said on Bloomberg’s "With All Due Respect." 

“To the best of my knowledge, this is the first time that a colleague has attacked, me. You’d have to ask Sen. McCaskill why.”

This story was updated at 3:55 p.m.