That said, the put-up-or-shut-up line works both ways. Romney could make the whole issue go away (and make Reid look foolish in the process) by simply releasing more tax returns. The campaign showed no willingness to do that, especially under the terms that Reid's established. But that seems likely to just invite even more speculation.

UPDATE: 8:46 p.m. -- Reid is not backing down. He put out a statement Thursday night declaring that his source at Bain (who he said told him about Romney not paying taxes) is "extremely credible" and insisted that the one way to clear up the matter is for Romney to be more transparent.