It is a pre-requisite for being on the b****t. Congress closed their eyes to that requirement and gave Obama an affirmative action pass. All potential candidates must register with the Federal E******n Commission. The FEC reviews the eligibility of each potential candidate according to the following basic requirements:

Age and Citizenship requirements - US Constitution, Article II, Section 1
No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President; neither shall any person be eligible to that office who shall not have attained to the age of thirty-five years, and been fourteen years a resident within the United States.

Surely you know that when the FEC realized that Obama did not satisfy the first requirement above, they bumped it to Congress for resolution.

Also note in the Constitution's listed requirements that "tax returns" are not among them.