As Mitt Romney edges closer to the Republican presidential nomination, the imperative grows for the former Massachusetts governor to release his income tax returns and disclose the identities of the fundraising bundlers who have brought in millions for his campaign. Mr. Romney’s determined lack of transparency on these two fronts — the candidate and his campaign have said he has no plans to release either — represents a striking and disturbing departure from the past practice of presidential candidates of both parties.

Asking candidates to make their tax returns public is undoubtedly an invasion of privacy. But it is one that comes with the territory of a presidential campaign. Such disclosure is not required by law but, as with the voluntary release of tax filings by the president and vice president, it has become routine, if at times grudging and belated.

Tax returns offer information not available on the financial disclosure forms that are legally required of candidates, including their charitable deductions and use of tax shelters. Tax information could be especially revealing in the case of Mr. Romney and his extensive investment income, which may be why he has been reluctant to release it. During his 1994 Senate race, Mr. Romney called on Sen. Edward M. Kennedy (D) to release his tax returns and show he had “nothing to hide.” Neither candidate released his tax information. Such secrecy will not stand for a presidential nominee.

The identity of a candidate’s bundlers is similarly important. Campaign finance laws limit individual contributions to a candidate to $2,500 per election ($5,000 if you include the primary and general election campaigns), but bundlers haul in tens or hundreds of thousands of dollars by tapping extensive donor networks. Knowing to whom and for how much candidates are indebted is essential information, of which candidates and their advisers are exquisitely aware. Yet under current law the only bundlers whose identities candidates must disclose are registered lobbyists. That information is useful but insufficient: A CEO who bundles $500,000 for a candidate can have as much influence as the company’s Washington lobbyist. Why should this knowledge be kept from voters?

In 2008, Mr. Romney promised to release information about bundlers but, aside from issuing state-by-state lists of finance committee members that lacked any details about money raised, does not appear to have done so. Now is the time to make up for that lapse.