I don't see any evidence of Trump ripping the party apart. Like at all.
The GOP politicians push back only on rare occasion, and then only in the most tepid possible ways. About 98% of the time, they rally around him and use the same talking points to spin everything he says and does, no matter how ridiculous, and no matter how clearly untruthful. So they are either liars or cowards or they legit support him.
Gallup's newest polls showed Trump with an 87% approval rating among GOP voters nationwide.
Make no mistake, the vast vast vast majority of Republicans are buying what Trump is selling and they will certainly double down in 2020. Will a few moderate Republicans peel away? ....maybe? But as the numbers show, it would be a sliver, not some sort of mass exodus.
The GOP is completely behind Trump. The people who vote Republican are overwhelmingly behind Trump. He's not splitting the party apart . . . he is the party. He embodies everything the party has come to stand for.
If we were looking for random patterns in polling to know whether he'll win re-election, he's had some of the worst presidential approval ratings out of any president since they started doing those polls.
Basically the absolute worst it can get is around 25%. That's the bottom. If Trump started hitting that number, the GOP would absolutely flip on him.
The only president's to go that low were:
Truman when unemployment doubled and the rising unpopularity of the Korean War
Nixon near his resignation
End of Bush W. near the peak of the 2008 financial crisis.
It looks to me that generally presidents stay if they are above the 45-50% range, and lost if they were below that.
Trump has hovered around 40% his entire presidency. He has never polled above 50%
The only one to buck that trend was Ford. He lost despite having a pretty good approval rating right around 45% Though from what I can read on the election it looks like it was kind of a Trump v. Clinton election. 2 highly unliked candidates. Which I think continues to be Trump's best bet at reelection: find the most blasé candidate, not going to bring out any first time voters, won't fire anyone up. (ie Biden, Klobuchar)
Trump is fighting a 60% disapproval. He just needs to keep 20% of those home to win.