"The bourgeoisie cannot win political power for itself nor give this political power constitutional and legal forms without at the same time putting weapons into the hands of the proletariat. As distinct from the old Estates, distinguished by birth, it must proclaim human rights, as distinct from the guilds, it must proclaim freedom of trade and industry, as distinct from the tutelage of the bureaucracy, it must proclaim freedom and self-government. To be consistent, it must therefore demand universal, direct suffrage, freedom of the press, association and assembly and the suspension of all special laws directed against individual classes of the population. And there is nothing else that the proletariat needs to demand from it. It cannot require that the bourgeoisie should cease to be a bourgeoisie, but it certainly can require that it practices its own principles consistently. But the proletariat will thereby also acquire all the weapons it needs for its ultimate victory. With freedom of the press and the right of assembly and association it will win universal suffrage, and with universal, direct suffrage, in conjunction with the above tools of agitation, it will win everything else.
It is therefore in the interests of the workers to support the bourgeoisie in its struggle against all reactionary elements, as long as it remains true to itself. Every gain which the bourgeoisie extracts from reaction, eventually benefits the working class, if that condition is fulfilled. And the German workers were quite correct in their instinctive appreciation of this. Everywhere, in every German state, they have quite rightly voted for the most radical candidates who had any prospect of getting in."
Stolen from a recent post by Doug Henwood.
Nah, Engels would disagree with you. The difference between Trump and Biden for a worker's movement/worker's party isn't material in the way Engels is talking about. Engels specifically sided with the bourgeoisie in completing their revolution, setting the stage for the working class to finally take control. Once that revolution was complete in Germany with bourgeoisie democracy, he realized that now all voting did was measure the readiness of the workers. One way or another the bourgeoisie would not relinquish their power via voting, so it wasn't worthwhile to focus on it. Hence "Universal suffrage is thus the gauge of the maturity of the working class. It cannot and never will be anything more in the modern state; but that is enough." Voting for Biden or voting for Trump isn't going to make a difference, since in Engels' understanding voting after bourgeoisie democracy is already established isn't a vector of change.
EDIT: Also note that the" it" that quote you referenced isn't the working class but the workers party. Bourgeoisie democracy is a necessary precondition for the worker's party to exist, but once it exists, or rather can exist (like the US right now), the time to side with the bourgeoisie is over.