Democrats have stepped up their calls for Mitt Romney to release more of his tax returns, clearly hoping for more embarrassing revelations about his overseas tax shelters or his long relationship with Bain Capital. Thus far, Gov. Romney has released his 2010 returns and estimated returns for 2011. But he has declined to release older ones.

Critics point out that Mr. Romney's father, George, set the standard for candidate tax disclosure when, during the Republican presidential primary campaign in 1967, he released 12 years...