Usually progressives of this sort won't actually say on what basis they think anyone who opposes them are ultimately right or wrong in principle, instead they have to rely on the same sort of emotional conditioning that their minds have been lost in. So here come the Nazis marching through the conditioned minds of this sort of half-wit, given that such a mind is conditioned to be indiscriminate it has to rely on the fact that everyone knows the Nazis were evil even if it cannot allow itself to go too far into discriminating why things are right or wrong.
It seems that what is left of the progressive mind reads a little like this in so far as it can be specified in actual thinking/language and reason: "I won't say that anything is good or evil because I've been conditioned to feel that would be a narrow minded kind of discrimination. It's downright intolerant. After all, the Taliban, the KKK and the Nazis have all said that some things are evil.... which proves my point that saying things are good or evil is....uh kinda wrong or somefin'!"
And so on, language and thinking itself seem to degenerate as such a weak mind is conditioned to be indiscriminate based on patterns of negative emotional conditioning which it tries to spread to others through the same methods and modes by which it was conditioned. Ironically this evil pattern (Evil? Oh my!) of emotional conditioning is found in the typical structure of Nazi and Islamist propaganda itself.
It seems that a mind that is conditioned to be indiscriminate and tolerant above all else must always lift evil up as the equal of good or tear good down to the equal of evil, ultimately it driven to seek equality. So this pattern of "thinking"/conditioning will have to portray old allies like the American and British as the moral equivalent of Islamists and Nazis with no regard for what is actually reasonable so that what is generally good can be portrayed as generally evil and vice versa. (A local example and a comment critical of critics, in so far as "It's just like somethin' or somethin'." imagery can be referred to as critical.)
It's ironic that progressives are typically ignorant enough to believe in the Darwinian creation myth and seem to derive much of their moral relativism from a Darwinian worldview which they believe to be scientific.
As the philosopher David Stove noted, if we agree that the Nazis were evil then such a worldview is associated with evil*:
It is less well known that...Adolf Hitler found or thought he found an authorization for his policies in the Darwinian theory of evolution. He said, for example, that "if we did not respect the law of nature, imposing our will by the right of the stronger, a day would come when the wild animals would again devour us--then the insects would eat the wild animals, and finally nothing would exist except the microbes. By means of the struggle the elites are constantly renewed. The law of selection justifies this incessant struggle by allowing the survival of the fittest. Christianity is a rebellion against natural law, a protest against nature."
...it is perfectly obvious that accepting Darwin's theory of a universal struggle for life must tend to strengthen whatever tendencies people had beforehand to selfishness and domineering behavior towards their fellow humans. Hence it must tend to make them worse than they were before, and more likely to commit crimes: especially crimes of rapacity, or of cruelty, or of dominance for the sake of dominance.
These considerations are exceedingly obvious. There was therefore never any excuse for the indignation and surprise with which Darwinians and neo-Darwinians have nearly always reacted whenever their theory is accused of being a morally subversive one. For the same reason there is, and always was, every justification for the people, beginning with Darwin's contemporaries, who made that accusation against the theory. Darwin had done his best to separate the theory from the matrix of murderous ideas in which previously it had always been set. But in fact, since the theory says what it does, there is a limit, and a limit easily reached, to how much can be done in the way of such a separation. The Darwinian theory of evolution IS an incitement to crime: that is simply a fact.
(Darwinian Fairytales: Selfish Genes, Errors
of Heredity and Other Fables of Evolution
By David Stove :106-109)
*Besides being evil, such a worldview is empirically groundless in the case of Homo sapiens because they are indeed sapient and so escape being governed by a law of natural selection at all times given that they are capable of intelligent selection. Darwinian reasoning can be rejected on the grounds that empirical evidence shows that it is not true and it can also be condemned as evil.