Trump could have won the Democratic nomination with his trade and foreign policy positions if he hadn't done the Birther schtick. He'd have to have run a different kind of campaign, however, and it would have had to be in a year where the Democrats felt a bit hard-pressed to win (e. g. 2004).
Also if he didn't base his campaign on building a wall, deporting illegals, and banning Muslims. Which are all
sine qua non of any Trump campaign. Also if he quit the climate change denial and the law and order rhetoric.
Strip those out, and you have "NAFTA sucks, and I'm gonna defeat ISIS somehow while also disengaging globally, disbanding NATO, and cuddling up to a Russian leader who is a consistent human rights abuser." I can't see him getting very far. He'd be a one-trick candidate.