This points out one of the reasons that Republicans in Congress are in no rush to impeach the President. If Trump is impeached, or impeached too soon, that will give Pence the advantage of incumbency, with three years or more to leave the stench of Trump behind him.
There are lots of Republicans in Congress who want the 2020 nomination for themselves, and are loath to surrender the advantage to Pence now. After 2018, Pence might be dismissed as a lame-duck caretaker President who would be owed no deference because of his office.
On the other hand, if Republicans fail to impeach Trump soon enough, the party faithful may turn on them in 2018. When a GM in baseball is trying to make a trade, there may be many fans who shout their support. Then, when the trade goes bad, nobody ever supported it.
The GOP leaders may know that Trump has tremendous support right now among Republicans, but they have to anticipate what will be the nature and extent of that support in the 2018 primary season, when incumbents who desert the President may face challenges from the right of their own party.
Tough situation to be in. Makes me almost want to shed a tear.