Mitt Romney said on 5/11/16:
“It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters.” “There is only one logical explanation for Mr. Trump's refusal to release his returns: there is a bombshell in them. Given Mr. Trump's equanimity with other flaws in his history, we can only assume it's a bombshell of unusual size.”
Since Trump said at a campaign rally on 1/23/16,
"I could stand in the middle of 5th Avenue and shoot somebody and I wouldn't lose voters", the real reason must be radioactive.
George Will speculates that
“Perhaps one more reason why we’re not seeing his tax returns is because he is deeply involved in dealing with Russia oligarchs and others.”
Mark Everson, IRS commissioner from 2003 to 2007 said, "It would be a real rupture in our political system," if a presidential candidate simply refused "to honor that transparency" and denied voters any chance to assess their taxes.
Fred Goldberg, IRS commissioner from 1989 to 1992 and practicing tax lawyer, said on 8/23/16: "The sooner Trump releases his entire returns, the better." "Inevitably, his refusal to release his returns raises a question: What is Trump hiding?"