He can't do it before the convention, because they are so bad, he might not get the nomination. However, once the convention is done and the rethugs are stuck with him, he doesn't have to release them because his base will still vote for him whether he releases the returns or not. I suspect his internal polling is showing that this issue is not affecting his base one iota. They hate Obama so much, they'd vote for Mitt (once nominated) even if he had already been convicted of tax evasion, and had accounts in Iran.
All they have to do is keep this think close in places like Ohio, Pennsylvania, Virginia, and Florida. The combination of suppression of of thousands of votes, and stealing elections where voting machines, supportive county clerks and Rethug Secretaries of State are in place, will ensure these races remain close. Then spend like a drunken sailor on his last leave to put him over the top. Rmoney thinks the Independents pay no attention until his ad bombs come out in the fall, and then are stupid enough to buy it all.
3) Throw in an Army of Mormons walking neighborhoods all over the country for good measure.
It might be just enough to overcome his own stupidity, nastiness, and greed.
1. Romney's strategy = lie and incite, rinse, repeat
Every attack levelled by Romney against Obama so far has been based on a bald faced lie or a distortion of fact. He targets his messages to "the base", the goal being to incite their anger and hatred and racism. He's probably got one BIG lie that he's going to roll out with a multi-million ad buy right before the election, hoping to swing enough voters his way to make it close enough that the vote-suppression and/or stealing will flip the election to him.