Potomac Watch: Bernie Sanders may have given up on the Democratic Presidential nomination but is determined to make Joe Biden unelectable. Image: Jonathan Ernst/Reuters

When it comes to “fake news,” we had a whopper this week. Voters were informed that Bernie Sanders was dropping out of the Democratic presidential contest. Joe Biden only wishes it were so.

True, Mr. Sanders announced on Wednesday the “suspension” of his campaign, noting that he trailed Mr. Biden by some 300 delegates, and that no “honest assessment” showed a path to the nomination. The Vermont senator said he could not “in good conscience continue to mount a campaign that cannot win.”

Yet Mr. Sanders said he nonetheless would (and could in good conscience?) “stay on the ballot in all remaining states and continue to gather delegates,” so as to exert “significant influence” over the party. He also declined to promise he’ll help Mr. Biden get elected. He instead blandly noted that his rival was a “very decent man, who I will work with to move our progressive ideas forward.”

This isn’t an endorsement; it’s a threat. The Democratic Party is split, and Mr. Sanders is the undisputed leader of its progressive wing. He’s not conceding gracefully; he’s not rallying Democrats behind a nominee; he’s not going anywhere—not without extracting a significant show of fealty from Mr. Biden. Put another way, the man who was too radical to win the nomination is now determined to make Mr. Biden unelectable.

Within hours of the Sanders announcement, newspapers were reporting that the two camps were in negotiations over which Sanders policies Mr. Biden would need to adopt to get Bernie’s blessing. The New York Times reported that the Biden campaign might begin rolling out these changes as early as this week. Up for discussion: climate, health care and student loans, for starters.

Concessions on policy aren’t all Bernie is demanding. The negotiations also involve discussions about Mr. Biden’s future cabinet, including which progressives will go where, as well as who cannot play a role. The left wants a Biden administration ban on anyone who has worked on or near Wall Street, the fossil-fuel industry, the health-insurance sector and the lobbying world, to name a few.

In a sign the entire Bernie universe has already seized on this hostage-taking strategy, a coalition of eight progressive groups sent their own open letter to Mr. Biden Wednesday, explaining that a campaign pledge of a “return to normalcy” wouldn’t cut it: “Going back to the way things were ‘before Trump’ isn’t a motivating enough reason to cast a ballot in November.”

The only thing that would make them support Mr. Biden, they write, is his agreement to meet their demands, which include endorsing the Green New Deal, Medicare for All, a 50% reduction in prison populations, a wealth tax, cancellation of student debt, free undergraduate tuition in public institutions, abolishing the filibuster, packing the Supreme Court, federal gun licensing, and abortion subsidized by federal taxpayers. Among the signers was Justice Democrats, the influential progressive outfit that was founded by former Sanders campaign leaders and supported the election of now-prominent voices like Rep. Alexandria Ocasio-Cortez.

This is dangerous territory for Mr. Biden, and it highlights the party divide that Mr. Sanders has inflamed. Mr. Biden has already shifted significantly to the left, looking to attract progressive primary voters. He now favors a health-insurance “public option,” free community college, huge tax increases and forgiveness of many student loans. Sewing up a nomination is usually the moment at which a candidate begins pivoting back to the center to appeal to independent and moderate voters.

Mr. Sanders’s intention is not only to block such a pivot, but to jerk Mr. Biden further left. That sticks the putative nominee with an impossible choice. Mr. Biden can maintain a “centrist” course and risk losing Bernie’s base. Or he can cater to Mr. Sanders’s extreme agenda and risk alienating independents, disaffected Trump voters, suburban women, blue-collar workers, etc.

He can’t do both—the policy gulf is too wide. And even should Mr. Biden make a few concessions now, there’s no reason to believe the pressure will end. Mr. Sanders says he’s taking his fight all the way to the convention, recently postponed to August. Some party leaders are so worried, they are discussing the possibility that a “virtual” convention could minimize Sanders dissent. Such a move risks infuriating the progressive base.

Polls show the top priority of a majority of Democrats is defeating Mr. Trump. But a majority is not all, and in a close election, party unity and enthusiasm are paramount. For many Sanders voters, ideology matters more than victory. Is the socialist willing to act as a spoiler? By the looks of this week, you bet.

Write to kim@wsj.com.

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