Nazis weren't really that popular until the beginning of WW2, just that the opposition parties hated each other even more and were unable to unite in opposition (a bit like the Russian revolution).
The conquest of France was an unifying (and legitimizing) moment when even the people who were passively skeptical of the regime mostly got behind it (actual opponents were almost all in exile or imprisoned by that point)
By the time WWII started, opposition was closed in literal concentration camps. There was heavy suppression of oth democratic and communist parties from the moment Hitler took power on 1933.
Also, the calls for opposition to unifie miss who the opposition was. As in, it was composed of both pro democratic and actual communist party that took directions from Stalin. Who were pro dictatorship too at that point.
I mean, it is popular talking point from from tankies. But, social democrats not unifying with actual communists despite Hitler propaganda claiming they are the same makes sense.
The conquest of France was an unifying (and legitimizing) moment when even the people who were passively skeptical of the regime mostly got behind it (actual opponents were almost all in exile or imprisoned by that point)