You're correct in that a popular vote wouldn't preclude figures like Trump from coming to office; and it isn't the point anyway since the reason for a popular vote is one of fairness rather than making liberalism undefeatable or whatever.
But I don't agree with calling Reagan a proto-Trump for similar reasons that Malkiah points out - while many of the changes leading to America's present state of political and economic paralysis occurred during his presidency, his conduct while in office was within the 'normal' bounds for American federal policy. The proto-Trump in the sense of a deeply unpleasant and highly corrupt individual that lies and cheats their way to success was actually Nixon.
Indeed, I'd argue Nixon's legacy is just as damaging as the specter of fascism Trump raised because Watergate was one of the key points in the 1970s that helped destroy America's trust in its public institutions and facilitate the privatization of its civil society.