Millions of voters will flee the Democratic party and enter the GOP.
like who?
anyways, the voters I think you are referring are already trump voters, so that is a mute point. However, democrats WILL make gains with minorities, since they will overwhelmingly likely turn out in higher numbers. Also, Trump isnt even that popular among Trump democrats, and most will flip in 2020 back to dems.
Whites, blacks, hispanics, and asians. Just to name a few groups. Great Presidents like Nixon, Reagan, and Trump get more voters the second time around.