Sen. Bernie Sanders (I-Vt.) speaks at a campaign rally at the Bon Secours Wellness Arena in Greenville, S.C., on Sunday. (Erik S. Lesser/European Pressphoto Agency)

Conservatives have always argued that the left believes in unilateral disarmament, but now we have proof: Sen. Bernie Sanders (I-Vt.) faces a primary opponent whose use of a private email server is under investigation by the FBI, but he refuses to attack her on the issue.

His failure to do so cost him victory in Iowa. It cost him victory in Nevada. And ultimately, it could cost him the Democratic nomination.

In the one state where Sanders has won — New Hampshire — exit polls showed 34 percent of Democratic voters said that honesty was the most important factor in their decision about whom to support. These voters chose Sanders by a stunning margin of 92 percent to 6 percent, helping put him over the top in the Granite State. By contrast, Clinton won by a wide margin among those who said the ability to win in November was the most important factor. But these voters made up just 12 percent of the electorate, not enough to make up for Clinton’s gaping honesty gap.

In Iowa and Nevada, however, a larger segment of Democratic voters put electability ahead of honesty. In Iowa, 20 percent said electability was their top priority and they chose Clinton by a margin of 77 percent to 17 percent. In Nevada, an even more Democrats — 25 percent — said electability was most important, and Clinton won them by a whopping 82 percent to 12 percent. She still lost to Sanders among Democrats who put a premium on honesty, but there were not enough of these voters to give Sanders a victory.

The lesson of the first three Democratic contests, therefore, should be clear: Clinton’s weakness is honesty, but her strength is her perceived advantage in electability. Knowing this, what must Sanders do to wrest the nomination from Clinton? Simple. He needs to exploit her weakness and undermine her strength — by putting a dent in Clinton’s perceived electability. The only way to do that is by raising the specter that Clinton’s legal woes could cost Democrats the White House in November.

Democratic presidential candidate Sanders targeted rival Hillary Clinton's Wall Street support at a campaign rally in Boston. (Reuters)

Fox News recently reported that the FBI is investigating not just Clinton’s use of a private email server, but also “whether the possible ‘intersection’ of Clinton Foundation work and State Department business may have violated public corruption laws.” And The Post has reported that the State Department inspector general had issued a subpoena “seeking documents about the charity’s projects that may have required approval from the federal government during Hillary Clinton’s term as secretary of state,” including records related to Clinton’s top aide, Huma Abedin, who “was employed simultaneously by the State Department, the foundation, Clinton’s personal office, and a private consulting firm with ties to the Clintons.”

Sanders should make this an issue. The very raison d’être of Sanders’s campaign is to challenge the Wall Street special interests and the pay-to-play culture in Washington.

Sanders does not have to join the Republican critics and say Clinton did anything illegal. He simply has to explain to voters that she has a major legal problem that could come back to haunt Democrats in November. All he has to say is: “If Clinton is the Democratic nominee and she ends up under indictment, the result will be a right-wing extremist in the White House.”

Sanders has won the hearts of the Democratic base, but he has to win their minds, too. Democrats already believe that he shares their values and would be a more reliable liberal in office. But they think that Clinton has a better chance of winning. He needs to disabuse them of this notion, by raising the very real possibility that voting for Clinton might lead to a Democratic disaster in November.

Thus far, Sanders has refused to do so. Amazingly, the issue of Clinton’s possible wrongdoing did not even come up in the last Democratic presidential debate. Can anyone imagine that if Donald Trump, Sen. Ted Cruz (R-Tex.) or Sen. Marco Rubio (R-Fla.) were having their actions investigated by the FBI they could get through a two-hour debate without either the moderators or their opponents raising the subject?

Sanders has said that he will not “politicize” the investigations. That is a mistake. If a majority of Democrats believe that Clinton is dishonest but electable, she will win. But if they can be convinced she is both dishonest and unelectable, she is toast.

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