Releasing one’s tax returns isn’t technically required when running for president, but it is a longstanding tradition that lets voters learn more about a candidate. When asked about the subject in a recent interview with Business Insider the Senate Majority Leader said, “For the last 30 or 40 years, every candidate for president has released their tax returns, and I think Donald Trump should as well.”

Hope Hicks, a spokesperson for Trump’s campaign, assured Fortune that Mr. Trump is undergoing a routine audit and will release his returns once it’s complete, reiterating a sentiment Trump himself has said many times. However, experts have told CNN Money that an audit doesn’t preclude disclosure.

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McConnell joins a larger group of prominent Republicans, including Mitt Romney, who say Trump should disclose his returns. Though he doesn’t support a Democrat-backed bill that would make it a legal requirement, a recent poll shows that most Americans do—despite the fact that both Trump and RNC Chairman Reince Priebus think they don’t.

There are various theories as to why the presidential candidate may not want people to see his tax returns—maybe he doesn’t donate to charity as much as he says he does; maybe he’s not as wealthy as he claims; and maybe he just doesn’t pay taxes—something we saw in his tax returns from the 1970s.

Whatever the reason, this is something that the Democrats will likely focus on in the general election and, as Fortune’s Shawn Tully speculates, delaying the disclosure could end up being disastrous for him.