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(CNN) —  

On Monday afternoon, as expected, the White House blocked former White House counsel Don McGahn from testifying on Capitol Hill about his time working for President Donald Trump – and what he told special counsel Robert Mueller about it.

That development was lumped in with a series of other moves by the White House to keep administration officials (past and current) from appearing in front of the Democratic-controlled House and, more broadly to resist any attempt by the House to investigate Trump and his associates.

And the McGahn move is that – but it’s also more. Here’s why.

McGahn was at the center of some of the most controversial and consequential moments of Trump’s first two years as President – most notably the alleged order Trump gave to McGahn to get rid of Mueller as special counsel in June 2017. McGahn told the special counsel that he had refused that order. He also told Mueller that Trump had later asked him to deny a New York Times report regarding the ordered firing, and that when McGahn refused to comply, the President interrogated him about his taking notes in their meetings.

Now, the really important part: Trump had repeatedly denied ANY of this happened.

“I was NOT going to fire Bob Mueller, and did not fire Bob Mueller,” Trump tweeted earlier this month. “In fact, he was allowed to finish his Report with unprecedented help from the Trump Administration. Actually, lawyer Don McGahn had a much better chance of being fired than Mueller. Never a big fan!”

Last month, Trump tweeted much the same: “As has been incorrectly reported by the Fake News Media, I never told then White House Counsel Don McGahn to fire Robert Mueller, even though I had the legal right to do so. If I wanted to fire Mueller, I didn’t need McGahn to do it, I could have done it myself.”

Someone is lying here. About a hugely significant moment. Now, that fundamental disagreement won’t be resolved by McGahn testifying on Capitol Hill. Trump will continue to tweet whatever he wants to tweet. And last time I checked, there’s no penalty for not telling the truth on Twitter.

But for something so critical as whether or not a) the President ordered his top lawyer to fire the special counsel looking into Russian interference in the 2016 election and b) if the President later asked McGahn to lie about the whole thing, it’s quite clearly in the public interest for McGahn to testify.

The Point: The odds are still very much against McGahn testifying – especially with the White House claiming immunity and insisting that he is under no obligation to heed a subpoena from the House Judiciary Committee (or anyone else). But whether or not McGahn will testify, it’s inarguable that he should.