It started with Mitt Romney, not Trump.
Romney repeatedly and blatantly lied about the impacts of his tax plan during the first debate with Obama in 2012. Obama struggled to deal with it, the comments of fact checkers after the debate didn’t help, the polls tightened, and everyone agreed that Romney won the debate. From that, the GOP clearly learned that lies can be sound political strategy.
Lies are, in fact, the basis for innumerable GOP commercials on pre-existing conditions. Will they work? We’ll find out next week.