It wasn't just the mob. Government insiders tried to keep Trump in power, too.

A common refrain among critics of a congressional investigation into the Jan. 6 attack on the Capitol is that there isn't anything more for the public to learn, so any inquiry is either a fishing expedition or cover for Democrats to keep it in the news. But Tuesday's disclosure by ABC News of a draft letter from a senior lawyer in the Department of Justice in December endorsing an attempt to overturn election results in Georgia should serve as evidence that a full-scale investigation is not just justified, but absolutely necessary.

What the letter makes clear is that top figures inside the Trump administration were willing to make patently implausible legal and factual arguments against the validity of Joe Biden's electoral victory — not because those arguments had any merit, but because they provided just enough of a fig leaf to keep Donald Trump in power. Put another way, the letter drives home that it wasn't just the president's supporters outside the administration who were willing to resort to extreme measures to prevent the peaceful transition of power; the call for a coup was coming from inside the house.

The Dec. 28 letter, which Jeffrey Clark — at the time the acting head of the Justice Department's civil division — apparently hoped to get signed by acting attorney general Jeffrey Rosen and acting deputy attorney general Richard Donoghue, included debunked claims about alleged improprieties in the counting of votes in Georgia. Clark apparently hoped to use those claims as the basis for the Justice Department to formally urge the Georgia legislature to call itself into special session (never mind that Georgia law doesn't allow such a maneuver), certify a competing slate of electors for Trump (never mind that the deadline for such a move had passed) and thereby give congressional Republicans a justification not just for contesting Biden's Georgia electors at the joint session of Congress on Jan. 6 but for seeking to have Georgia's electoral votes counted for Trump.

The letter is indefensible. Its factual analysis relied upon theories about Georgia's vote counting that had already been disproved, including by Georgia's Republican secretary of state. Its constitutional analysis rested on the absurd — and novel — notion that the U.S. Constitution, as opposed to state law, has anything to say about when state legislatures can convene themselves without gubernatorial approval. Worst of all, the letter just ignores the fact that, under relevant federal election laws, the die had been irrevocably cast. Georgia had certified its slate of presidential electors by Dec. 8 — the deadline imposed by the Electoral Count Act of 1887. On Dec. 11, the Supreme Court turned away Texas's analytically incoherent lawsuit against Georgia and three other states Biden won that had been a last-ditch effort to prevent their duly certified electors from casting their electoral votes. And on Dec. 14, Georgia's only duly-certified electors had met, as required by law, and cast their 16 electoral votes for Biden. Despite the letter's baldfaced misrepresentation to the contrary, no other duly certified Georgia electors had met Dec. 14. Thus, as even Vice President Mike Pence understood, by Dec. 28, there was no remaining legal mechanism available to prevent the Jan. 6 Joint Session of Congress from certifying Biden's victory.

And even if none of that were true, and there was a universe in which Clark's machinations could actually have come to fruition, the best possible result for Trump would've been Congress either giving Trump Georgia's electoral votes or refusing to count Georgia's electors at all. In either event, Biden still would've won. By Dec. 28, there were no legal means available to extend Trump's presidency; there were only illegal ones.

But imagine how the letter would have been received by Trump's supporters had it actually been signed by the acting attorney general and released. The Justice Department throwing all 150 years of its institutional weight and credibility behind claims of fraud in Georgia? Behind legal claims that the fraud could be remedied before Jan. 20? Behind the broader narrative that the election had been "stolen" from Trump? Given the rhetoric and discourse that we saw even without the nation's chief law enforcement officer publicly endorsing such a letter, it's not hard to imagine the calls that would have followed its release for similar efforts to un-ring the electoral bell, whether in Arizona, Michigan, Nevada, Pennsylvania or Wisconsin. Whether those efforts succeeded or not, the letter would have touched off a full-blown constitutional crisis — and, it's not hard to imagine, violence on a far larger scale than we saw Jan. 6.

Fortunately, neither Rosen nor Donoghue took the bait. Donoghue, in fact, responded several days later that "there is no chance that I would sign this letter or anything remotely like this," according to the ABC report. But what if Trump had fired both of them and named Clark as his acting attorney general — a move he clearly had the legal authority to make, and which several contemporaneous news reports suggested he was considering? As the New York Times reported Jan. 22, every single Justice Department official besides Clark apparently agreed to resign if Trump attempted such a maneuver — prompting Trump to stand down and leave DOJ alone. So we dodged a bullet, but the letter drives home just how close we came to things being far worse — and just how much figures inside the Trump administration, and not just the former president's supporters outside of it, were involved in efforts to undermine the peaceful transition of power. That's an enormous story unto itself.

We know about this story only because ABC News obtained the draft letter and published it. There's no official record to rely upon; there's no witness testimony under penalty of perjury to confirm the New York Times's reporting; there's no independent endeavor to identify how many other government officials were involved in this misbegotten, anti-democratic enterprise — or how close it came to succeeding. All we have is media reports. And for as much as we're better off with those reports than without them, the fact that we're only now learning about Clark's letter drives home why a deeper investigation is needed.

Was Clark an outlier, or is he the only one who made the mistake of committing his plotting to paper? That's the imperative here: to produce an comprehensive narrative documenting efforts from both within the Trump administration and without to prevent Biden from being inaugurated Jan. 20. It seems like we all have a compelling interest in finding out — not just for history's sake, but to try to prevent a similar attempt in the future.

Steve Vladeck is a professor at the University of Texas School of Law, co-editor in chief of Just Security, co-host of the National Security Law Podcast and a CNN legal analyst.