I was skeptical Trump would deliver the sort of nominee(s) conservative SCOTUS voters wanted, because his personal philosophy of constitutional interpretation — if we should even dignify it with that label — is nothing like the small-government originalism or textualism they favor. But I was wrong. Trump seems to have heeded the Republican establishment here. He’s turned out three nominations very pleasing to this portion of his base: Justice Neil Gorsuch for those with a libertarian edge, Justice Brett Kavanaugh for the executive authoritarians and national security hawks, and now Barrett for the social conservatives (in terms of cultural cachet, at least; her actual bench record is more complex and shows a civil libertarianism that seems to place her between Kavanaugh and Gorsuch on this measure).
Once Barrett is confirmed, then, the transaction is complete. The SCOTUS voters got what they wanted (a 6-3 conservative majority on the Supreme Court), and Trump got what he wanted (the presidency). The deal is done.
And given that fact, why should SCOTUS voters support Trump again in 2020? They don’t owe him anything. They paid in full in 2016, and now he’s held up his side of the bargain, too. What else is there to say? You don’t hang out with the car salesman after you’ve signed the papers. You certainly don’t pay him twice the sticker price for providing precisely what he promised. This is how a transaction works, and the Trump-SCOTUS voter transaction is over. Electing him again won’t make Gorsuch, Kavanaugh, and Barrett somehow more confirmed to the Supreme Court than they already will be.