The odds are pretty close that he'll either resign or be impeached before the end of this term. Discussed it with one of my contacts in the field, and he told me to check the bookmakers odds. 10/11 impeachment or resign. I don't think he'll get impeached before the 2018 election, but I see a major turnover in Congress that would open the door to impeachment. I don't think he'll resign and quietly go away, either. He strikes me as the type that will be more than happy to take the party down with him.
Generally, the GOP carries about 23% of the registered voters. Usually, the DNC has about 30%. After Nixon resigned, the number of Registered Democrats ballooned to 70%+. By 1980, it returned to normal.