Then, the impeachment would require the Republicans to support the motion. Better yet, it would even be more a sign that the movement to challenge was initiated by the Republicans. Let's see when Mueller's report will come out if it will be condemnatory enough to incite Republicans to do the right thing and evict Trump.
That said, if Trump is accused, Mike "I think smoking does not cause cancer" Pence would be the president, and one may wonder if it would not be like changing 4 quarters for a dollar. After all, there is a possibility that Trump chose him as a career partner precisely because blushing the obese idiot would not end up changing much.