Republican officials are exploring how to handle a scenario that would be unthinkable in a normal election year: What would happen if the party's presidential nominee dropped out?

ABC News has learned that senior party officials are so frustrated — and confused — by Donald Trump's erratic behavior that they are exploring how to replace him on the ballot if he drops out.

So how would it work?

First, Trump would have to voluntarily exit the race. Officials say there is no mechanism for forcing him to withdraw his nomination. (Trump has not given any indications that he no longer wants to be his party's nominee.)

Then it would be up to the 168 members of the Republican National Committee to choose a successor, though the process is complicated.