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The cautionary tale now engaging progressive, Democratic forces in the face of a probable Donald Trump presidential nomination has been the widely noted George Wallace presidential campaign, the 1968 Democratic National Convention in Chicago, and the law and order reaction that followed. Among others, Todd Gitlin in The Washington Post and Michael Cohen in The Boston Globe go to 1968 to ruminate about their fears concerning the bully Trump. The more frightening, but perhaps more instructive case is the German federal election of November 1932-the last free and democratic election held there until 1949.

Listen up Hillary Clinton and Bernie Sanders supporters, and pay close attention, those who think of themselves as being to Sanders's left: History will judge us sternly if we fail this moment.

Earlier in 1932 Adolf Hitler's Nazis had become the largest party in parliament, with about 37 percent of the popular vote. The Social Democrats (SPD) and Communists (KPD) together had about 36 percent of the vote, but were in fierce, mortal competition. The traditional German nationalists could not form a stable government without so they called new elections. Now, in November, Hitler lost seats and the combined vote of the SPD and the KPD was larger than his, as were their combined parliamentary seats. While the KPD was hostile to the Weimar arrangements, it nevertheless proposed to the SPD a common front against the early 1933 power grab by the Nazis. The hostility between the two working class parties with egalitarian visions was too deep.

The split that had created the Bolshevik-Menshevik divide in Russia and had divided the German working class parties over the Great War now prevented a united front against Hitler.

The Communists characterized the SPD as "social fascists"-no better than the Nazis. In a similar vein, leftist commentator Chris Hedges and others have recently written that Hillary Clinton is no better than Donald Trump: "Voting for Clinton and supporting the Democratic Party will not halt our descent into despotism."

The SPD had, of course, invited such "praise" by suppressing the KPD in the '20s (and was complicit in the murder of its inspirational founders Rosa Luxembourg and Karl Liebknecht).

Looking backwards, fatalistically, many historians view the notion that the two parties could have cooperated against the rise of Nazism as unrealistic: One committed to parliamentary and constitutional means, the other "revolutionary" in inspiration. Yet it was the communist dissident Leon Trotsky who sniffed the rotten end of rationality in all this when he called upon worker militants to unite with the SPD against fascists. And after January 1933, the Communists did unsuccessfully propose united action with the SPD against the Nazi seizure of power.

If left leaning activists are serious about their characterization of Trump as a fascist, a comparison recently made by former Labor Secretary Robert Reich, then they better get serious about the problem of unity. It is all very well for Hedges to tell us to vote with our feet in the streets, but as the last century teaches us, the ability of authoritarians to resist popular movements is robust. After the election of November 1932, Hitler came into office as Germany's largest party (though a minority). And while working class parties together neared a majority, they were two divided to oppose him. The center parties-ostensibly committed to constitutionalism-succumbed to a fear of Hitler's retaliation and a fear of the left. They cooperated in the formation of a government that would ultimately eliminate them.

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The German elections in March 1933 were held under repressive and violent conditions and the concentration camps were next.

If one really thinks Trump is a danger (or for that matter Cruz!) then the problem of unity, voter turnout, and solidarity is the great historical task before us. Tom Hayden was been particularly articulate about this. It is not fun. The Clintons will not grasp warmly the hands of Sanders; and the young (and not so young) Sanders militants will not be so eager to vote for a Clinton. But as Karl Marx long ago noted, we don't get to make history as we please "under self-selected circumstances, but under circumstances existing already, given and transmitted from the past."

The principals themselves have a lot of responsibility here. Sanders may come to the Democratic convention with a very large contingent of enthusiastic supporters-whose enthusiasm will be required for a Clinton general election victory. Keep in mind that a Trump nomination could very well turn out previously absent white voters. Both Democratic leaders need to ensure near historic turnouts of constituencies that compose their greatest strengths: young lefties for Sanders, African Americans for Clinton. The two contenders for the Democratic nomination should be easier to unite than were the warring German parties in 1932. An explicit, transparent statement of common goals would go a long way toward a win in November.

There is some value in thinking about 1968. Unable to countenance support for the administration which had made war in Vietnam, I advocated a boycott of the Humphrey-Nixon contest, while friends in California and elsewhere chose Eldridge Cleaver over Humphrey. In the meantime, the law and order backlash against our demonstration tactics in Chicago also helped Nixon win a razor thin victory over Humphrey. What we got was more bombing in Vietnam, Laos, and Cambodia and a shameful episode of presidential abuse of power.

Fidel Castro once said in a courtroom that, "history will absolve me." And what will it do to those who now hold themselves aloof from allies at the precipice?