We don't actually know if Richard Nixon ordered the break-in to the Democratic National Committee headquarters in the Watergate complex on June 17, 1972.
That, to me, is still the most remarkable feature of the Watergate crisis. A president was forced out of office for the first time in American history by a scandal centering on a single crime, and we still don't know if he actually ordered it.
In his memoirs, Nixon denies it, though he smugly adds, "I could not muster much moral outrage over a political bugging." Jeb Magruder, a dirty tricks operative for Nixon, revealed three decades later that he had overheard Nixon and his reelection chair John Mitchell planning the burglary. But as historian David Greenberg notes, "Mr. Magruder had [previously] discussed that same meeting without noting Nixon's participation." Dirty tricks operatives aren't the most reliable of sources.
We don't even know why it happened — if the burglars were looking for evidence that the DNC was receiving money from the North Vietnamese or Cuban governments (as conspirator Howard Hunt insisted), or information embarrassing to White House counsel John Dean (as G. Gordon Liddy, who planned the break-in with Hunt, claimed), or, as another popular theory has it, trying to find out how much DNC chair Larry O'Brien knew about Nixon's financial dealings with billionaire tycoon Howard Hughes.
But what we do know, the "smoking gun" that eventually forced Nixon out of office, was that Nixon ordered his chief of staff to get the CIA to force the FBI to abandon its investigation into the break-in.
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That was enough.
Some Republicans had stood by Nixon through his firing of the independent counsel investigating the matter, through multiple aides and Cabinet officials resigning, through the White House's effort to resist subpoenas for documents and tapes. But when the "smoking gun" White House tape was released on August 5, 1974, Nixon's remaining support from Republicans evaporated. Two days later, Senate Minority Leader Hugh Scott (R-PA), House Minority Leader John Jacob Rhodes (R-AZ), and former presidential candidate Sen. Barry Goldwater (R-AZ) went to the White House and informed the president that he had no support left in Congress.
They were shocked and horrified that Nixon had personally participated in the cover-up; before then there was still a sliver of a chance that the president himself wasn't part of the conspiracy. They told Nixon that, now that his role in the cover-up was known, the votes were there to impeach him and remove him from office. The day after that, the president announced his resignation.
Similarly, there's a lot we don't know about Trump and his campaign's ties to Russia. We know that the FBI and other agencies have been looking into any contact Trump's campaign advisers Paul Manafort, Carter Page, and Roger Stone might have had contact with the Russian government during the election. We know that intelligence agencies suspect those three might have worked the Russian officials to coordinate the release of hacked emails. We know that disgraced former National Security Adviser Michael Flynn, White House senior adviser and Trump son-in-law Jared Kushner, and Attorney General Jeff Sessions all lied about or failed to disclose communications with Russian Ambassador Sergey Kislyak.
We don't know how all these pieces fit together. We don't know exactly what Donald Trump Sr.'s direct involvement is, or how aware he was of his advisers' efforts, or the nature of his business relationship with Russia. That's provoked a lot of very valuable investigative journalism, as well as a lot of outright conspiracy theorizing.
But focusing too granularly on the details of Trump's personal involvement risks setting the bar too low for him. It risks suggesting that unless we find undeniable proof of collusion between Trump and the Russian government, he's in the clear.
The fact of the matter is that without any more information than we already have, we already know Trump's conduct is almost as outrageous as what Nixon acknowledged in the smoking gun tape.
In Nixon's case, what crossed the line, moving top leaders from his own party to go to the White House and tell Nixon that his presidency was over, was Nixon's attempt to hamper the FBI's investigation into Watergate.
And we know, for a fact, that Trump fired FBI Director James Comey because he was upset by the FBI's investigation into his Russia ties.
We know that because Trump said so himself. Asked by NBC's Lester Holt why he fired Comey, Trump replied, "I said to myself, I said, 'You know, this Russia thing with Trump and Russia is a made-up story, it's an excuse by the Democrats for having lost an election that they should've won.'"
If Trump means that he fired Comey purposefully to derail or obstruct an FBI investigation, Notre Dame law professor Jimmy Gurulé writes, his actions could constitute obstruction of justice, a felony. (There's still a sliver of a chance he didn't; as University of Chicago law professor Aziz Huq noted, maybe Trump was just "annoyed that Comey wasn't toeing the party line that Russia is no big thing.") But the evidence keeps mounting. Reports that Comey had requested more funds for the Russia investigation and that grand jury subpoenas against Flynn's business associates were issued in recent weeks suggest that the White House was concerned the investigation was going too fast.
This kind of information is what forced Nixon out of office and shocked his Republican allies out of defending him. For now, Trump's Republican allies in Congress are standing by him and refusing to consider even an independent prosecutor, let alone impeachment.
But this is not a "where there's smoke there's fire" situation. We don't need to know much more to know that the president has committed conduct that was once thought sufficient to warrant removal from office.
The Comey firing isn't smoke. It's fire.
Commentary by Dylan Matthews, a senior correspondent at Vox. Follow him/her on Twitter @dylanmatt.
For more insight from CNBC contributors, follow @CNBCopinion on Twitter.