Rush Limbaugh radically transformed the Republican Party. He elevated conservative media into a coequal branch of party politics, and pioneered a style of rhetoric, argument, and entertainment that would come to define conservative politics.
The things we now think of as particularly Trumpian features of conservatism — the insults, the conspiracies, the blend of entertainment and politics and anger — Limbaugh had been doing it for a quarter-century before Trump showed up to the party.
He did what the first generation of conservative media activists had failed to do: he made right-wing media entertaining, profitable, and politically powerful. Though he was building on their work, he also transformed it.
He often bent the GOP to his will: his immense popularity when he arrived on the national scene in 1988 confounded GOP politicians, who chose to court him for fear of crossing him.
From George HW Bush putting him up overnight in the Lincoln bedroom to Michael Steele begging his forgiveness, he was a a precursor to the kind of loyalty politics on the right with which we're now so familiar.
He infused his show with racism and misogyny, wrapping both in jokes and satire so he could claim that critics were taking him too seriously. But it was a consistent feature of his show for decades.
Anyway, there's much more to say, but for now I'll just end by noting that the trends you've seen in the GOP over the last five years have all been visible on Limbaugh's show for decades — not just bc he prefigured them but bc he helped create Trump's Republican Party.