Bernie Sanders is no longer campaigning against Hillary Clinton. He's campaigning against Donald Trump and the ghost of Ernst Thälmann.

Ernst who? Thälmann was the leader of the German Communist Party from the late 1920s until the Nazis arrested him a few months after they took power in 1933. But to the degree that anyone remembers Thälmann today, it's for the absurd and calamitous party line-handed down by Joseph Stalin, but which Thälmann ensured that all German communists would echo-that he and his comrades advocated during Hitler's ascent to power.

In the last years of the Weimar Republic, the real menace to Germany, Thälmann argued, wasn't the Nazis but the Communists' center-left, and more successful, rival for the backing of German workers: the Social Democrats. The SDs, he said, were actually "social fascists," never mind that they were a deeply democratic party without so much as a tinge of fascism in their theory and practice. But as the Communists' rival for the support of the German working class, the SDs became the chief target of the Communists' campaigns.

Thälmann and the entire party machinery accused the Social Democrats of an endless stream of betrayals of working-class interests. These betrayals ranged from the non-existent to the compromises the Social Democrats had struck with more conservative parties in coalition governments, to the SDs' opposition the Communists' program and tactics. In 1931, the Communists actually worked with the Nazis-whom they termed "working people's comrades"-in a failed effort to bring down the Social Democratic government of Prussia, Germany's largest state.

While the Communists were more commonly not in alliance with the Nazis-who, after all, subjected party members to a reign of terror even before they seized state power-they continued to insist that the Social Democrats were no less fascistic, and no less dangerous, than the Nazis-a position that Thälmann continued to propound as late as the election of March 1933, which ratified Hitler's hold on power (he had been appointed chancellor two months earlier).

Indeed, the Nazi regime, Thälmann argued, should not vex leftists, as the Communists would quickly overthrow it.

"After Hitler, Us!" was the Communists' slogan throughout 1932 and early 1933, until Hitler arrested or killed all its leaders.

In a sense, Thälmann was right. After Hitler's death in 1945 and the Nazi surrender, the Communists, through the strength of the Soviet army, did come to power in East Germany. By then, of course, close to 60 million people had died in World War II and the Holocaust, and Thälmann himself, at Hitler's command, was killed in Buchenwald in 1944.

Thälmannism, then, is the inability (be it duplicitous, willful, fanatical, or just plain stupid) to distinguish between, on the one hand, a rival political tendency that has made the compromises inherent to governance and, on the other hand, fascism. And dispelling that inability is precisely what Bernie Sanders will be doing between now and November.

I'm neither equating Donald Trump with Hitler nor saying he's fascist in the classic sense. Trump has no organized private army of thugs to attack and intimidate his rivals, as both Hitler and Mussolini did. But Trump's racist, xenophobic, and nationalist appeals; his division of the nation into valorous and victimized native-born whites and menacing non-white interlopers; his constant employment of some Big Lies and many Little ones; and his scant regard for civil liberties make him the closest thing to a fascist of any major party presidential nominee in our history.

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Yet a minority of Sanders's supporters fail to grasp the threat that a Trump presidency poses to the nation-to immigrants, to minorities, to workers, and even to the left and to themselves. I doubt more than a handful will actually vote for Trump, but Jill Stein and even

Gary Johnson will win some of the Sanders diehards' votes (though for voters, moving from Medicare-for-All Sanders to Medicare-for-None Johnson requires either extraordinary ideological footwork or simple brain death).

In states where the race between Clinton and Trump is close, however, a Sanders diehard's vote for Stein or Johnson, or a refusal to vote at all, is in effect a vote for Trump.

In endorsing Clinton on Tuesday, Sanders conveyed three messages. The first was to vouch for Clinton herself: He praised her long commitment to winning universal health care and to the embattled working and middle class; he also praised her intelligence (a not sufficiently commented upon point of distinction between Clinton and Trump).

The second was to point out to his followers how much he and they had moved the Democrats, and Clinton with them, to the left-on the minimum wage, on increasing Social Security benefits, on free tuition at public colleges and universities, on stricter Wall Street regulation, on free trade in general and the Trans-Pacific Partnership in particular. Maintaining his clout by staying in the race until this week, he won the inclusion of previously contested positions in the Democratic platform, establishing in the process a close working relationship between officials of his campaign and Clinton's. (Contrary to most of the media coverage, he also won on the Trans-Pacific Partnership: The amendment adopted by the platform committee, which was written by the AFL-CIO, repudiated the particulars of the TPP, such as the private courts where corporations can sue signatory nations over regulations they don't like, while, in deference to President Obama, not condemning by name the document whose contents it rejected.)

Sanders's third message was to contrast Clinton's generally progressive positions and record with Trump's menacing diatribes. Most of the contrasts he drew, in classic Bernie fashion, were on economic issues, but he also raised the issue of Supreme Court appointees and the power the justices will hold over Americans' fundamental rights and whether the government will be a democracy or a plutocracy. It's these latter distinctions, I think, that he needs to dwell on most if he's to win over the last of his diehard supporters, who see Clinton as so inherently flawed they're willing to risk a Trump victory. Sanders needs to make clear what he touched on at the endorsement rally: how much a Trump presidency threatens democracy itself. He needs to confront a renascent neo-Thälmannism head on. It was a cataclysmic error in the 1930s; it would be a cataclysmic error today.