From a British/European (and possibly global) perspective, the American left is not ineffective, but non-existent and, apart from a few eccentrics, always has been.

I have looked through the ideas of Bernie Sanders and Alexandra Ocasio-Cortez - both accused by Americans at large of socialism - and cannot find a single policy that would not be entirely welcomed by Boris Johnson or Margaret Thatcher (of accursed memory). There is no mention of universal, tax-funded, free-at-the-point-of-use healthcare. No mention of public ownership of utilities and strategic industries. No State Pension. No intention to repeal the Second Amendment. No tax increases on the wealthy.

Basically, the US has one hard-right party - the Democrats - and one extreme right party - the Republicans. The only recent change is that the Republicans have lurched even further right from ordinary plutocracy (government of the poor people, for the rich people, by the rich people) to the edge of fascism. The 'extreme left' (ie well right of centre) of the Democratic party are what we would call 'One Nation' conservatives, convinced that it is still possible to 'level up' the less well-off without taking anything away from the hyper-rich.

The best thing that could happen to America is for Britain to become the 51st State!