Two basic facts govern impeachment and removal of a US President:
- It requires a simple majority of the House to impeach (specifically, for the House to refer Articles of Impeachment to the Senate).
- It requires a 2/3 majority (or 67 members) of the Senate to then remove the President from office after a Senate trial.
Let’s say that Mueller presents a report showing that Trump colluded with Russia. The Republican base is polled, and effectively decides that they would rather stick with Trump (especially given a highly favorable Fox news coverage press).
The House goes ahead and prepares a report, the Senate holds a trial and votes not to remove Trump from office.
This scenario — impeachment but no removal — is the absolutely worst possible outcome for anyone opposed to Trump. Trump can claim that the entire affair was a witch hunt, a meaningless waste of taxpayer money, etc. — and would then be effectively completely released from any possibility of oversight by Congress. Barring something completely beyond the pale (e.g. actually, literally shooting someone), that’s it for Trump oversight.
For the sake of argument, let’s say that Trump is actually impeached and removed from office. In that scenario, you wind up with a President Pence. The rest of the line of succession isn’t actually any better. President Paul Ryan?
If the Democrats successfully take the House in 2018, theoretically the new Speaker of the House would take over if both Trump and Pence were simultaneously removed from office. It’s hard to imagine a scenario where Republicans in the Senate would possibly go along with any vote that would lead to removing Trump and Pence and giving the presidency to, say, Nancy Pelosi. It’s possible to imagine Trump being impeached/removed, but then Pence would select a new Vice President, even if he were impeached/removed shortly thereafter.
So, best case scenario, your most likely outcome for a Trump impeachment/removal is President Pence.