I'm sure Trump grasps these ideas and so long as he pleases his base and Mitch McConnell fears being the primaried along with other Republican Senators, the prez has nothing to fear.
The Republicans are stuck in a catch 22 of their own making. In the latest Gallup poll, Trump remains 90% approval among Republicans, 33% among independents, and 4% among Democrats. In primaries, it's a deadly combination for any incumbent who opposes Trump, but in a general election, it's a deadly combination for anyone who supports Trump.
According to Pew, party affiliation breaks down into 26% Republican, 31% Democrat, and 38% independent. I assume the remaining 5% refused to answer and could probably be considered Independent to a large degree. Of the Independents, 13% lean Republican, 7% are true Independents, and 17% lean Democrat.
So going into an election, a Republican candidate can count on about 39% of the vote will vote for them if they support Trump, but the remaining 60% mostly loath Trump and would really have to hate the Democratic candidate more to vote for Trump, and that's highly unlikely this time around.
There are many states and districts where the electorate is so Republican that the real race is in the primary. There the calculus for candidates is to stay on Trump's side or get primaried from the right.
I do think that some of the Democrats aiming for the presidency would be better off going after their state's senate seat instead. Beto O'Rourke should go after Cornyn's seat in Texas. If the economy falters between now and the election (which is highly probably from just how the business cycle works combined with the damage Trump and the Republicans did to the economy), a Democrat with a national profile running for the senate in a red state has a chance. Stacy Abrahms similarly would have a shot at unseating David Purdue in Georgia. Winning the presidency is all for naught without the Senate and House too. There are a lot of red Senate seats up for re-election, but most are in safe red states.
There are some places where Democrats could pick up seats. Some in places that were not seriously possible a few years ago. Susann Collins and Cory Gardiner are in serious trouble. But Kansas and Arizona had some upsets in the midterms and that puts Martha McSally at risk and puts Roberts' seat in Kansas in play. In Iowa, the disastrous trade war has turned the state against the Republicans, but the state GOP are still rabidly pro-Trump. That puts Jodi Ernst in one of the bigger catch 22 situations. She's already known as kind of a nutter in a state that is overall tired of nutters, but with a GOP that still loves them. If the Democrats can put up a good candidate against her, she'll be in trouble.
2020 could become a re-alignment election when a new party system starts. We're in the characteristic political chaos at the end of a party system. A Democrat with truly new ideas can sweep in a new constituency like FDR and Reagan did and set a new narrative for a generation or more. I haven't quite seen the new pattern gel yet, but there are noises.
If 2020 does turn into a realignment election, there will be a number of pattern changes to the vote. We could see a number of reliable red states flip. Anyone who has any awareness of voting patterns knows that Reagan's key to victory was flipping the South pretty solidly red. It had been safe Democratic territory since before the Civil War. But there were other patterns that shifted too. FDR had driven the Republicans back into the Northeast and had locked up the rest of the country. But the Pacific Coast states were not reliable for the Democrats and often went Republican. In 1976 Ford won the western US and Carter the east with a few scattered exceptions. Interior western states voted Democrat in many presidential elections.
The Republican cult is currently strong and Fox News has them believing BS, but their hold could snap if reality does penetrate into the bubble. And ex-cult members are not kind to the cult. Fox News' is still the top network in cable news, but their audience is slipping fast. MSNBC will probably overtake them this year. One problem Fox News has is their audience is mostly over 60. In 2015 the median age was 68 and I'm sure it's older now. The Republicans are the Shakers of cults. Their dying out, literally.
The young conservatives have a high percentage of mean louts who could turn violent, but they are really a pretty small percentage of the population. Basically they are highly concentrated nasty in a small container.
Another wild card is what happens to News Corp and Fox News when Rupert Murdoch dies? Two of his sons will control News Corp after he goes and neither of them are very conservative. Since Roger Ailes left Fox, they have allowed the on air talent more leeway to criticize Republicans, but they can't be too loose or their father would disapprove. If Rupert Murdoch were to die before the 2020 election, we could see Fox News completely change, which would throw the cult into confusion.
Something could happen to Trump between now and the 2020 election too. There are several possible scenarios where he's not on the ticket in 2020: defects to Russia to avoid legal troubles, is indicted and SCOTUS allows him to stand trial, his health collapses or he dies, or possibly something else we haven't thought of yet. Several things that never happened before happen every week now.
The Republican party post Trump is something not even the deepest insiders in the GOP knows what is going to happen. It will likely become a Game of Thrones scenario with different factions playing to win control of the party at all costs.
