The US has blocked social democrats from positions of power multiple times, not the least of which was the democratic national convention blocking sanders (an imperialist socdem in 2016), and henry wallace (an anti-imperialist socdem) in the 1930s. Both of these candidates were the most popular of the bunch, but the DNC is a private organization and can hand pick who it wants.

And you're right, even if a socdem was elected, they can't do anything: the US is a bourgeois democracy ( ie a capitalist dictatorship ), and capital controls and stands above the political system.

Democratic socialism isn't possible not only in US, but anywhere. At least as long as USA power exists. But even regardless US even the best demsocs who actually have power never really try to implement socialism.