Lesser evilism
Time and again, supporting the lesser evil has produced the greater evil. In 1932, German Social-Democrats supported the bourgeois candidate Paul Von Hindenburg against Hitler, and Hindenburg appointed Hitler as chancellor. In 1936, the CNT-FAI allied with the bourgeois republicans, rather than forming an independent working-class front with the revolutionary UGT, and the government repeatedly sabotaged the revolution including by attacking anarchists in May the following year. In summary, lesser evilism didn't even make sense even in these extreme cases, when workers faced the threats of Adolf Hitler and Francisco Franco.
Further reading
Hal Draper, "Who's Going to Be the Lesser Evil in 1968?
Vernon Richards, Lessons of the Spanish Revolution
Lance Selfa, The Democrats: A Critical History