There are a series of connections between Donald Trump and his closest advisers and Russia that at least raise significant questions. Some of those questions could be answered if Donald Trump was willing to release his tax returns, but he's unwilling to do that.
Richard Nixon released tax returns when he was under audit.
You [Republicans] are not going to cut taxes. You're going to raise taxes on the middle class.
Donald Trump started this campaign in 2014 he said if I run for president I'll absolutely release my taxes.