The US doesn't have a social democratic party though and never had social democrats in power. The problem in the US is not about social democracy but about a fundamentally broken political system which in practical terms bans any other party which isn Democrats or Republicans.

Sure social democracy has had challenges in the Nordics, but all the core social democracy institutions are still solid and present. The social democratic parties themselves may not reign supreme anymore but much of their ideology has been accepted by other parties. Free higher education, health care, affordable child care, generous sick leave, public pensions etc are not going away.

If you look at the statistics you will see that most Nordics have managed to keep wages growth for the working class in line with economic growth. In the US wages for the working class has been quite stagnant for decades.

Whether you are a social democrat or market socialist we still face the same fundamental problem that what was the natural voting base no longer exist. Modern work life has become very complex. But I see in Norway that there are movement towards re-imagining social democracy and socialism on the left.