Sen. Bernie Sanders (Vt.) is proving to be a formidable challenger to Hillary Clinton for the 2016 Democratic nomination, but Fox News contributor Charles Krauthammer believes only one thing can stop Clinton's run to the nomination — and it's not Sanders.

Speaking to Fox's Bret Baier Tuesday night during the networks's Super Tuesday Two coverage, Krauthammer explained that only an "FBI primary" could stop Clinton's head-of-steam.

"[Sanders] cannot win this nomination one-on-one against Hillary," Krauthammer said. "The only thing that stops her is what we've talked about: the FBI primary."

An "FBI primary," as Krauthammer previously explained in January, is a situation where FBI Director James Comey "decides" who wins the Democratic nomination, in light of the controversy — and possible criminal wrongdoing — surrounding Clinton's use of a private email server as secretary of state.

According to Krauthammer, if Comey decides to proceed with a criminal referral of Clinton, then her campaign would likely collapse, sending Sanders — or another Democrat — to the party's November ticket.

In the absence of a decision from Comey's FBI, Krauthammer added that Clinton is sure to win her party's nomination.

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