If Meuller forces Trump to hand over his tax returns then I'll predict that Trump will resign rather than expose his financial dealings. We know for certain that Trump has Loans outstanding to Deutsche Bank and given Deutsche Banks egregious dealings in the money laundering business they will have no problem disclosing their activities with Trump. 
Trump Jr.  has admitted that Trump corporation has done much business in Russia over the years. 
So for Trump it's always been about winning and looking the part. Legally, he can call Meuller anything he wants but if he fires Meuller then it's clear he has something to hide. Washington doesn't like this mess and the establishment will make sure Trump is gone. 
What's worse is Trumps affinity for Russia. Americans literally hate Russians and for Trump to fawn over Putin, that's not acceptable. 
This has nothing to do with politics. It's one's  allegiance to the flag. 
Trump hasn't played a very good American.