Trump has spent a lot of time trashing people for not paying what he felt was their fair share of taxes. He has gone after many businesses very publicly for paying taxes but not enough in his eyes. He's also threatened businesses with retribution due to taxes.
Then suddenly it becomes more and more obvious he doesn't pay any himself. No skin in the game while trashing millions of other people just scraping by for not paying anything.
I think this may be more damaging to him than you are willing to acknowledge.
Perhaps..his rabid base won't blink, and for the undecided's, I think the vast majority of them already believe that he pays no taxes and that he's hiding his returns because of that. This is really no surprise to them. Trump's working class supporters don't support him because they think Trump is "one of us", an honest working class guy. They support him because he's a rich fat cat who wants to get richer and fatter...and will bring back their jobs in the process. Although, if your still "undecided" on who is unfit to be president this far in, anythings possible.
Yes..it does make him sound like a hypocrite. I seem to remember him saying: "Barack Obama who wants to raise all our taxes, only pays 20.5% on $790k salary."
From the response from the campaign:
"Mr. Trump knows the tax code far better than anyone who has ever run for President and he is the only one that knows how to fix it."
Lol..."I crash my car a lot, and I know the best mechanics to take it to for repairs. So put me in charge of your automobile company"