There's 1 big problem with Donald Trump's tax plan

mnuchin trump tax reform corporate tax rate_00002922
mnuchin trump tax reform corporate tax rate_00002922

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    Mnuchin outlines massive business tax cut

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Mnuchin outlines massive business tax cut 01:29

(CNN)Treasury Secretary Steve Mnuchin was riding high on Wednesday afternoon. He was in the midst of unveiling President Donald Trump's tax proposal, a plan that he had promised would be "the biggest tax cut and the largest tax reform in the history of our country."

Then ABC's Jon Karl asked Mnuchin about whether Trump would reconsider his refusal to release his own tax returns. It got awkward. Fast.
It goes quick. (Mnuchin didn't want to answer.) So, here's exactly what he said:
"The president has no intention....The president has released plenty of information and has given more financial disclosure than anybody else. I think the American population has plenty of information on his taxes."
    This is simply not true.
    Trump is the first president not to release his tax returns in four decades. That is beyond dispute.
    Trump has said the reason he won't release his tax returns is because he is under audit. But there is no law or even rule that would prohibit someone under audit from releasing his or her returns. In fact, Richard Nixon did it in 1973.
    There has been some dissonance in that explanation, most notably from White House counselor Kellyanne Conway, who said this in January:
    "We litigated this all through the election. People didn't care. They voted for him, and let me make this very clear: Most Americans are very focused on what their tax returns will look like while President Trump is in office, not what his look like."
    Trump, too, has been pressed on whether Americans care to see his tax returns. "I won. I became president. I don't think they care at all," he said in a news conference.
    Trump: People don't care about my tax returns
    Trump: People don't care about my tax returns

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      Trump: People don't care about my tax returns

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    Trump: People don't care about my tax returns 00:47
    White House insiders have insisted that the audit was the real reason the returns weren't being released. But Mnuchin also seemed to suggest that there is no interest in Trump's taxes from the public and, therefore, they wouldn't be released.
    Polling tells a different story. A January CNN/Opinion Research Center poll showed that almost three-quarters of the public -- 74% -- thought Trump should release his tax returns. Two-thirds said that Trump "should disclose more ... about his financial connections and business interests."
    The point is this: Mnuchin tried to dismiss Trump's refusal to release his tax returns as an unimportant side issue in the broader conversation about the tax reform package. It's not.
    Trump's lack of transparency on taxes gives Democrats a ready-made -- and politically effective -- reason to oppose whatever legislation arises out of this draft. "If Donald Trump can't show us whether this tax plan will benefit his own bottom line, I can't be for it," is an easy way for any Democrat -- even those in swing districts -- to justify their opposition.
    For Republicans, Trump's unwillingness to release his returns puts them in a bit of a bind. They will want to talk exclusively about the plan's merits and why it makes sense of the American people. But they won't be able to do that without facing the question of why Trump isn't willing to level with the public about how his family business would be affected by the tax package. Plus, there are already a decent number of House and Senate Republicans on record as calling for Trump to release the returns.
    Far from a side issue, the Trump tax returns loom as a giant potential distraction for congressional Republicans as they begin to try to build momentum behind the bill.
    Mnuchin doesn't seem to get that. Yet.