FDR actually kinda did do that. He broke the Washington precedent and ran for President four times, he scared the Supreme Court shitless to the point where they signed off on blatantly unconstitutional land grabs against Japanese emigrants, and the Democratic Party was able to ride high on the fumes of the Progressive movement for decades afterward.

We don't think of him as a dictator, because a lot of what he did was ultimately reforms necessary to maintain America as a republic. The alternative would have been Nazi America. But he was still exercising dictatorial power, and he was responsible for massively increasing the power of the Presidency as a result. Hell, part of the reason why Trump is so dangerous is specifically because of the damage FDR did to the checks and balances on the Executive Branch.