There is no system that is immune to takeover from a demagogue. There's not even any hard evidence that any system is more resilient to it than the US is. It's all just tradeoffs.

Germany had 7 major political parties in the run up to 1933. In fact if you look at the history of dictatorships that took over democracies, having 2 to 3 stable institutionalized parties is actually protective. The other thing that appears to be protective is a history of peaceful transitions of power, which the US has the longest or second longest.