From Addicting info, an opinion I cannot disagree with:
A Trump nomination will drive the racist base into a frenzy and they will surely come out to vote their hate but the large number of moderate Republican voters (yes, they still exist) will either vote against Trump or hang their head in shame and stay home. On the other hand, the Democratic base will flock to the voting booth in abject horror to keep Trump as far away from the White House as possible.
The combination of low conservative votes combined with potentially historic liberal turnout would not only result in the Republican Party losing the White House in a landslide, but also losing races all down the ballot, even ones that should not have been seriously contested. The Senate would be a total write off. A handful of critical Governorships would be at risk. Several state legislatures would be seriously at risk. Even the House would be in play despite the extreme gerrymandering Republicans have done.
Trump will not go quietly into that good night. He will scream at the top of his considerable lungs and the media will be there to eat up every second of it. There will be a million articles (like this one!) written about how the Republican Party told its base to sit down and STFU, essentially telling them that they should leave the big decisions to the grown ups.
The base, Trump’s rabidly racist and anti-establishment followers, will not just shuffle their feet in disappointment. If they don’t outright quit the party and form their own, they will run primary challenges against any Republican that does not join Trump in condemning the party. They certainly won’t vote for the hand-picked nominee no matter how much they promise to be like Trump.
After that, the schism in the party will be irreparable. The base will never support the establishment again and without that support, Republicans will have to appeal to moderates, something Republicans are laughably bad at doing and which will only further alienate their base. The best move for the party would be for them to let Trump run, lose spectacularly and then use that loss to discredit the extremism that led to Trump in the first place. But that would require long-term thinking and Republicans sacrificed their ability to do that when they embraced the Tea Party back in 2009.
Here’s how there is an out for the GOP: Use whatever influence they have now and open up the electoral process to so-called third parties. Give the right-leaning radicals a place to go. And at the same time, the progressive wing of the Democratic parties would have a place to go and abandon the Dems is roughly equal numbers.