A source close to the Romney campaign told Politico that the decision was made to release Mitt Romney’s 2011 tax return “so that there would be some distance between that headline and the debate, and to do the best the campaign can with an issue on which voters have likely already absorbed a lot of information.”
“There was also no ideal time for the release of the information, but Friday was after five days spent discussing remarks Romney made in a secretly taped fundraising video about 47 percent of Americans considering themselves ‘victims,’ and not paying taxes. Providing information about his own tax rate seems geared toward eliminating the question about whether he himself has at any point not paid taxes, but without providing the returns themselves.”
Think Progress: 10 questions Romney should answer about his taxes.

