Sen. Claire McCaskillClaire Conner McCaskillMcCaskill welcomes ninth grandson in a row Dem group launches M ad buy to boost vulnerable senators Senate Dems block crackdown on sanctuary cities MORE is accusing the media of treating Sen. Bernie SandersBernard (Bernie) SandersDems ponder gender politics of 2020 nominee 2020 Dem contenders travel to key primary states After Florida school shooting, vows for change but no clear path forward 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 ClintonWoman behind pro-Trump Facebook page denies being influenced by Russians Trump: CNN, MSNBC 'got scammed' into covering Russian-organized rally Pennsylvania Democrats set to win big with new district map 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.