The Clintons' Tax Returns

On what grounds could they continue to withhold them from the public? Novak makes a useful point:

Clinton's tipping point may have come when it was announced that her $5 million loan to her campaign came from a fund she shares with Bill Clinton. That puts into play for the general election business deals by the former president that transformed him from an indigent to a multimillionaire and might excite interest in their income tax returns, which the Clintons refuse to release.

Don't the Dems need a candidate who is vetted properly?