In the run-off second round of the German presidential election in 1932, the German Social Democratic Party (SPD) called for votes for Hindenburg, a "traditional" right-winger, against Hitler -- just as in the previous presidential election they had backed Wilhelm Marx, the candidate of the Catholic-bourgeois-liberal Centre Party. Their catchcry was that workers must support the "lesser evil". A few months later Hindenburg appointed Hitler as Chancellor.
In his writings on Germany, aimed mostly at people in or around the CP, Trotsky largely takes it as obvious, not needing detailed argument, that the SPD vote for Marx or Hindenburg was unprincipled from a working-class point of view. In a later article, on Spain (14 September 1937), when he feels that not so much can be taken for granted in the way of his readers holding certain assumptions, he explains more.
Specifically, he argues that siding with the Republic in the Spanish Civil War was quite different from voting for Hindenburg.