The premise is inaccurate. Reagan had plenty of political experience by 1980, and what's more at that point had developed into one of the most skilled political operators in US history. He'd served 8 years as an successful and highly popular governor of a major state (California) that at the time was a GOP stronghold, winning in landslides. He nearly threw off an incumbent President in 1976 in the nomination that year: that's a near impossible feat, no matter how weak the President is. Nobody was under any illusions as to who the nominee would be in 1980 by the later Carter years, and the fact that Reagan would put an end to America's weakness regarding the Soviet Union was, far from being a problem, a key selling point. The Republican Establishment ended up providing the VP candidate.

There is no resemblance whatsoever to Trump, a complete political amateur, albeit one who is proving in his own way to be a skilled campaigner. In fact, Trump's biggest legacy so far is (on the GOP side) pretty much finishing what Dubya started in destroying the post-1976,, Reaganite consensus in totem.