I know that a party not supporting a sitting POTUS is somewhat inconceivable, but does anyone think this guy is going to get a second term?
Depends on the state of the economy. If unemployment is low, I think it's likely he will.
The problem with attacking a sitting president is that he has his own faction, and Donald Trump still has a faction among "outsiders". The party insiders recognize him as a clown, but the people who voted for him still actually like him. Oh, sure, his cavorting with porn stars is an embarrassment, but he cut taxes and he doesn't like foreigners, so he's ok in their book.
In addition to his own faction, he has all the people who just don't want to rock the boat. Together, they could easily make up enough to get the nomination. Then, once the nomination is secure he could easily be facing a really awful Democratic candidate. They've nominated a few of those. If the economy is good, plenty of people will stick with Trump.
A gigantic GOP midterm loss could also have an effect, but I don't see that happening yet. There's a long time between now and November, but if the election were held today they would face no worst than typical midterm losses. The party with the White House usually loses some seats in the midterms, and this time might be the same, but I don't think it is looking like a huge blowout. If there is a huge blowout, someone might rise to challenge Trump from inside the GOP.