Can’t do much without a mass party to back you up. It takes a lot of people.
That’s why you pull a reverse Sinema. Lie your ass off and pretend to be the most corporate corpodem that ever sold out. You’ll only have to fight one major party instead of both of them to get elected, then you can move as far to the left as you want, so long as you avoid small aircraft.
Even there, though, even as a president, you can’t meaningfully move much without the support of the party. The difference for fascists is that the Republican party legitimately has a ton of actual fascists, so fascist presidents are extremely dangerous. If a Socialist led the DNC, it would be spoiled and wouldn’t have much of a chance to move left.