I wouldn't let this go if I were Mitt Romney.

Any unauthorized release of information on a person's tax returns is a federal offense with a penalty of up to $5,000 and/or a 5 year prison sentence, and if that violation is committed by any officer or employee of the United States, he shall, in addition to any other punishment, be dismissed from office or discharged from employment upon conviction for such offense.

Harry Reid is making allegations against Romney that are either lies, or if true have been leaked to him or by him.

Either way this is a serious breach of the public trust and an investigation should be launched to find the truth of this.

If I was a key member of the Romney campaign I would be before every media outlet I could find and demand this. This is a bigger story that whether Romney releases his pre 2010 returns, which he has already released.

26 USC § 7213 - Unauthorized disclosure of information | LII / Legal Information Institute