Bernie Sanders -- Democratic enemy No. 1? (AP Photo/Charlie Neibergall)

Bernie Sanders is "extreme," and if the media were to dig into him a little more, he wouldn't be doing as well as he is.

That attack comes not from a Republican, but one of the Democratic senators with whom Sanders caucuses.

"The media is giving Bernie a pass right now,” Sen. Claire McCaskill (D-Mo.) said Thursday on MSNBC. “I very rarely read in any coverage of Bernie that he’s a socialist.”

First of all: Wrong. It's hard to find an account of Sanders's personal politics that doesn't include the s-word. It's all over the place.

And the Vermont senator is indeed a socialist. He's an independent who caucuses with the Democrats in the Senate. He's also running for president as a Democrat, and several polls show him gaining a bit on frontrunner Hillary Clinton -- whom McCaskill supports -- while making headlines for drawing thousands of people to campaign events as far off as Denver.

McCaskill argued Sanders wouldn't be doing so well if the media treated him like other candidates. "I think everybody wants a fight," she continued.

That might be true. But in saying that, McCaskill has essentially set up one. Her comments ring of the kind of intra-party snipes you'd expect to see in a contested, genuinely competitive primary. Witness her start comparing Clinton to Sanders:

"I think Bernie Sanders has a message that's touching people," McCaskill said. "And I totally get that. But so does Hillary Clinton."

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McCaskill said Sanders is "too liberal" to win, and she could be right.

As The Fix has pointed out several times, it's too soon to know whether Sanders's impressive support this early on will translate into votes that could actually give Clinton a run for her money. The other day, 92 percent of Democratic-leaning voters said they'd be comfortable with Clinton as the Democratic nominee. And she's still polling like a frontrunner; about 50 percent of Iowa Democrats told Bloomberg last week she's her first choice, while 24 percent said Sanders is.

Sanders told Bloomberg later that day this is the first time he thinks a Democratic colleague "has attacked me like this."

And he has better hope it isn't the last, because it's a sign that Clinton supporters are at least mildly concerned.

More Bernie:

Why that Bernie Sanders boomlet is less than meets the eye

5,000 people came to hear Bernie Sanders speak in Denver. And that means ...

What primary? 92 percent of Democrats are comfortable voting for Hillary Clinton

Bernie Sanders: From hippie migrant to would-be president