Additons/corrections eagerly asked for
What happens if Trump withdraws:
As someone who has suggested all along that there'd be some chance that either Trump ir Clinton might be replaced as nominees due to a variety of circumstances, it hardly surprises me that (this early) it now seems possible that Trump might withdraw. I'm a political junkie of long standing, worked in elections, was a poli sci major. Not an election lawyer, so I will stick to more vague procedures that I am sure of.
1) No one can force Trump to withdraw. He is the nominee. There is no impeachment procedure, although it is possible based on state law some state parties might be able to put someone else in (nnot sure about this).
2) If he withdraws, his replacement would be made by the 170 or so members of the Republican National Committee. The convention is over and done with. I suppose they could poll delegates and get their advice, but the choice is the committees.
3) The VP candidate of course could be considered, but has no special claim.
4) The VP candidate is in place. He can't be replaced unless he withdraws. He was nominated by the convention.
5) The new presidential candidate would be chosen by ballot(s), with whomever gets a majority being the nominee.
6) In terms of timing, the earliest deadline for getting on a state ballot is sometime after Labor Day. It varies by state, but nearly all have Sept deadlines.
7) Anyone can be nominated. It doesn't have to be someone who ran.
8) If it happens after state deadlines, the candidate remains on the ballot. But remember presidents are chosen by electors, not voters - we are electing only electors who will choose the president. Thus if someone is no longer around, the electors are free to vote for the replacement (in legal terms, irrespective of state law, electors can do whatever they want).
9) Ted Cruz came in second, and would make a huge push. TO go with a lower placing candidate than him (Kasich, Rubio, Bush et al) would be tricky.
10) My guess if he wants it the replacement would be Paul Ryan. He is the highest ranking Repub official, did run for VP last time.
11) Trump would have no say in the replacement, except possibly for basing his withdrawal on approval of replacement (though he'd have to withdraw before he was replaced.
12) The same procedure would be in play for the DNC, where I assume the candidates would be Sanders, Kaine and Biden
We have one semi-precedent for this. Thomas Eagleton left the ticket as VP in 1972 on the Dem side. The DNC followed McGovern's advice and named Sargent Shriver. This I believe was in August.