Progressive journalist Ryan Grim said Monday that the Democratic Party fears that a potential presidency for Sen. Bernie Sanders (I-Vt.) represents a threat to the party's establishment.

"A Sanders's presidency is a threat of an entirely new party of which you're not necessarily welcome, or you might be welcome in but in extremely subservient and much less paid role," Grim, The Intercept's Washington, D.C., bureau chief, told Hill.TV.

"That, I think, mingles with a genuine fear that Bernie Sanders isn't electable, and they'll run with whatever sticks," he added. "It's too early to say what exactly is going to stick because they've never thrown the whole kitchen sink at him before."

Sanders and his allies have repeatedly claimed that the Democratic Party is against his nomination. These fears were exacerbated in November when former President Obama reportedly told advisers in private that he would speak up if necessary to stop Sanders from becoming the Democratic pick.

Sanders has consistently polled near the top of the 2020 presidential primary.

According to the RealClearPolitics average of national polling, the Vermont senator places second with 19 percent support behind former Vice President Joe Biden's 28 percent. The two are followed by Sen. Elizabeth Warren (D-Mass.) with 15 percent and South Bend, Ind., Mayor Pete Buttigieg with 8 percent.

Every other candidate is polling at 5 percent or lower.

Meanwhile, on Monday Sanders released letters from three doctors showing he is "fit and ready to serve as president" after suffering a heart attack earlier this year.

-Tess Bonn