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The DSA's rapid growth in numbers (from around 5,000 members before 2016, to nearly 90,000 in December 2020) and in workplace and neighbourhood activity was opened up by the Sanders campaigns. That shows that it is possible to build socialist organisation through efforts in Democrat primaries, and not only with Sanders but with others like Alexandria Ocasio-Cortez. Given the anomalous structure of US politics, it also shows that it is possible to do this without giving any blanket commitment or deference to the bourgeois common run of Democrat politicians.
The most likely path for the development of a workers' party in the USA is through something like what has been called a "dirty break" from the Democrats. That means the socialists combining direct socialist work in workplaces, unions, and neighbourhoods with running socialist candidates in Democratic primaries, supporting those socialists when they win the primaries, sometimes running socialist candidates directly in their own name; and a perspective of building forces through that combined work to create a real third party.
In an election choice between two bourgeois candidates, where there is a big and clear difference — such that in rational comment observant socialists routinely say that a victory for one is much preferable to a victory for the other — and where there is no viable working-class candidate, it is a valid option for socialists to vote critically for the "lesser evil". They explain this simply by saying that it is the "lesser evil", and that encouraging workers to come out and vote in such a choice (while also urging them to work with socialists in our efforts to create better choices in future) helps future possibilities better than a shrug or a non-vote. That applied in the November 2020 US presidential election.