(Let's leave aside the suspicion that no one in McCain campaign higher than an intern did anything more with Romney's tax returns than put them in a box on the shelf. Romney was never seriously considered as McCain's running mate.)
In 2008, the real question was: "Was it legal?" Did the candidate pay the amount of taxes that the law required? At the end of the Bush administration, it was assumed, even admirable, that one would take advantage of every possible legal means to minimize paying income taxes.
In 2012, we are much more tuned in to the question of the inequality of wealth and income. Occupy brought the 99:1 ratio into popular consciousness. Romney's tax returns are now a vivid example of how the ultra-wealthy play by different rules, how they make and protect their wealth from the common obligations of society. The scandal is that whatever Romney paid, it WAS legal, even if he paid nothing on 20 million dollars of income. Romney's tax returns will be a case study in how our economy was legally looted by speculative, financial capital.
The sea change in our politics is this lens of the 99:1 ratio, which comes from the Occupy movement. Inequality, not legality, is the key issue.
Romney wanted this election to be a referendum on Obama's economic performance. Occupy changed the terms and so, the election is shaping up to be instead, a referendum on the 1%'s mismanagement, exploitation and looting of the nation for the last 32 years.