With millennials, Democrats have enjoyed roughly a 20-point advantage in party ID since Pew starting asking them the question 14 years ago. Millennials are now the largest living generation and are starting to vote in greater numbers. This is a huge problem for the GOP that both predates and is far bigger than Trump.
But the data gets worse for Republicans the deeper you dig into it. In 2016 exit polling, for instance, 18- to 24-year-olds went more heavily for Hillary Clinton than their older millennial counterparts, suggesting that, if anything, the Republican position is falling apart with the tail end of the millennial generation. Had complacency and the wreckage of the bitter Democratic primary not led many millennials to vote for Jill Stein or Gary Johnson, those numbers would likely look much worse.
And it gets even worse for the GOP: A 2017 Pew poll found that fully a quarter of young Republicans had defected to the Democrats since 2015.