Hello hvilleherb,
It is well known that some Trump supporters would support him even if it is proven that he is a dirty dealer. They simply don't care and would look the other way. Look at all they have accepted so far!
Regardless of that fact:
75% of the Republican Party is not enough to maintain political power. It took everything they had to just barely squeak by with the 2016 election. And even then, the Democrats had 3 million more votes.
Also, it is highly likely that if serious charges come out that many will turn on Trump. Those possible charges are not known at this time, so current polls cannot show what would occur in that event.
It is entirely likely that if serious charges come out, and tentative supporters begin to run away from the president, that the president could be impeached. That could happen before the election. Can you say 'blue wave?'