While Barr's letter has been criticised for being too friendly for Trump, including by members of Mueller's team, there's a real argument to be made that it set the unhelpful expectation that Mueller had somehow exonerated Trump.
And despite his own claims of total exoneration, Trump has continued eviscerating the Mueller investigation. That suggests that he's girding for some potentially bad news tomorrow.
What might that bad news be? Below are a few possibilities.
1) Trump's actions were very obstruction-y, but he's the president
The biggest question heading into tomorrow is what Mueller says about obstruction. That's because this is the crime Mueller chose not to expressly clear the President of, according to Barr's letter.
That's unusual. Generally speaking, prosecutors either decide to charge or not, and leave it at that. But Mueller decided to add that qualifier.
Here's how Barr characterised it:
"After making a 'thorough factual investigation' into these matters, the Special Counsel considered whether to evaluate the conduct under Department standards governing prosecution and declination decisions but ultimately determined not to make a traditional prosecutorial judgment. The Special Counsel therefore did not draw a conclusion one way or the other as to whether the examined conduct constituted obstruction. Instead, for each of the relevant actions investigated, the report sets out evidence on both sides of the question and leaves unresolved what the Special Counsel views as 'difficult issues' of law and fact concerning whether the President's actions and intent could be viewed as obstruction. The Special Counsel states that 'while this report does not conclude that the President committed a crime, it also does not exonerate him.' "
The question is why Mueller chose this path. Did he do it because the evidence was compelling but perhaps not conclusive - because he believes the actions could be obstruction but that he simply couldn't decide? Or did he just decided to punt on the question because Justice Department guidelines say Trump can't be indicted anyway?
Assuming Mueller expounds upon his reasoning here, that will be key. The former would mean the evidence could be pretty damning, but perhaps not damning enough - including to cost Trump much politically. That latter could be worse for Trump, in that Mueller could describe actions that would be considered criminal for anybody else, but Trump's status as a sitting president would mean the decision was left up to Congress.
2) A narrow verdict on 'conspiracy'
Ever since the Barr letter came out, more than a few smart people have pointed to Barr's and Mueller's specific choice of words when it comes to interactions between the Trump campaign and Russia. Specifically, they never use the words "collusion" and instead say there was no "conspiracy" or "coordination."
Barr's letter summarises Mueller's report by saying:
"As the report states: '[T]he investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities.' "
And then in a footnote, Barr says:
"In assessing potential conspiracy charges, the Special Counsel also considered whether members of the Trump campaign 'coordinated' with Russian election interference activities. The Special Counsel defined 'coordination' as an 'agreement - tacit or express - between the Trump Campaign and the Russian Government on election interference.' "
The Washington Post's Philip Bump wrote a couple of weeks back about how these distinctions could be significant. Essentially, it could still mean that there was some evidence of collusion more broadly - as House Intelligence Committee Chairman Adam Schiff, D, continues to argue - but that it didn't rise to the level of actually coordinating and conspiring. Perhaps information was shared and assistance was encouraged, but there was no actual, specific agreement.
Government watchdog Larry Noble summarised this point nicely: "Defining coordination as requiring an agreement between the parties creates a massive hole in the wall against foreign corruption of our elections. For example, does Mueller believe that, unless he can prove there was an agreement to do so, it is not illegal for the campaign to provide information to the Russians, such as polling data, to help them target their ads supporting Trump and opposing Hillary Clinton? What if there were meetings between Trump campaign officials and agents of the Russian Government where information about the Russian and Trump strategies to get him elected were exchanged, discussed, and its use encouraged, but there is no proof of an agreement as to what specific activity will take place?"
Criminality matters, and the fact that Mueller appears to have found no crime here matters greatly. But what if he found something that could be read as amounting to the broader concept of "collusion" with a hostile foreign government? That could still be politically problematic, depending upon how serious and how clear the evidence.
Or we could simply be parsing all of this too closely. Mueller might say it really wasn't even a close call - and that's why he made the call on this one, even as he didn't on obstruction.
3) It could embarrass or expose those around Trump - and cause turmoil
NBC News laid out the stakes of the report not for Trump, but for those around him:
One person close to the White House said there is "breakdown-level anxiety" among some current and former staffers who cooperated with the investigation at the direction of Trump's legal team at the time.
There is also concern that new facts in the report could be disclosed that do not reflect favourably on the President, two people familiar with the discussions said.
"You have a whole bunch of former White House officials and current White House officials, but especially former White House officials, who were told to cooperate," the former White House official said. "So people went and did that, and now the uncertainty is just how much of that information is going to be in that report and how identifiable to individuals is it going to be. And nobody knows."
Another person familiar with the discussions said the officials who are worried are those who said negative things about Trump. This person said the "million-dollar question" swirling around Trump world is how much of the report will be redacted, specifically if it will be a "net plus or minus" 100 pages of the more than 300-page report.
This White House has leaked like a sieve, in part because people inside it and the Administration seem to be so concerned about how unwieldy and unmoored the entire operation is. But it's one thing to make the decision to voluntarily leak to a journalist; it's another to be compelled to tell the truth to investigators.
To the extent this provides detail about the inner workings of the White House and what those around Trump truly think about him and various actions that have been taken, that could paint a very unsavory picture.
While Trump might not be accused of crimes, what if it becomes clear that those around him were attesting to the reported chaos or describing his actions in details that reflect poorly on the commander in chief? What if former White House counsel Donald McGahn's 30 hours of interviews prove truly embarrassing for Trump, in some way? Stephen Bannon's reportedly said of the 2016 Trump Tower meeting with a Russian lawyer: "Even if you thought that this was not treasonous, or unpatriotic, or bad s--t, and I happen to think it's all of that, you should have called the FBI immediately." What if others say similar (if less colourful) things?
And what happens when people are fingered - by Trump or other White House staff - as having been the sources of derogatory information. The White House has already been a place rife with competing factions willing to throw others under the bus; a juicy Mueller report could put that internal feuding on turbo.
