In the Weimar Republic, KPD and NSDAP parties representation in the Bundestag, as distinct parties with relatively large representations.
In 1932, the KPD had 360k members, 5m+ votes, and 14-17% of the electorate. From 1928, the KPD was basically the Stalin party. They believed that the SPD (most analogous to modern-day "Democrats" in USA) were fascist, let alone the NSDAP.
There is no modern equivalent to the KPD in contemporary American politics. The most left-leaning elected officials in the Bundestag-equivalent would have been SPD members. The fringe militant people on the left in the USA do not have any political representation at the federal level, mostly because they do not exist in meaningful numbers, certainly not anywhere approaching 10-17%.
I think that 10-17% might exist on the right (it is probably closer to 5%), but especially after this year, it will become clear that the Republican Party (more analogous to the CDU, historically) is not their party.
If history is cyclical, Trump will form his equivalent of the DNVP shortly, which will not be radical enough for the people storming the Capitol today. This election and transfer of power might be the American Dolchstoß moment, in retrospect. I really hope we've learned.