We have to assume that whatever damaging information is in there is fairly current. What could it be? That he claimed no charitable deductions? Everyone already knows he is a miser, so that wouldn't be big news. That he claimed huge charitable deductions for giving money to his own foundation? New York State is already on that case. The most plausible thing that would show up on a tax return and would be damaging would be transactions involving Russia, such as deductions for interest paid to Russian banks for loans, capital gains or losses on Russian real estate deals, etc. These would undercut his statements that he has no business with Russia.
Not grounds for impeachment. But certainly enough to hurt him politically in areas where he is already vulnerable.
They told me if I voted for Hillary Clinton the president would be emotional, impulsive, and unpredictable. They were right. I voted for Hillary Clinton and got a president that is emotional, impulsive, and unpredictable.