Robert Mueller could testify to Congress as soon as next week, as House Democrats have proposed a May 15 appearance for the special counsel. President Donald Trump, meanwhile, sounds as if he’d prefer that it never happen, tweeting Sunday that Mueller shouldn’t testify and that it would constitute a “redo for the Dems.”

But just how big an event might this be? And does Trump actually have anything to fear?

When this prospect was first raised long before the Mueller report came out, I wrote that people should temper their expectations. Mueller is, as anyone who knows him will tell you, a by-the-book prosecutor who isn’t terribly interested in creating a media spectacle focused on himself. If Mueller wasn’t going to say it in the report, I argued, it’s not like he was suddenly going to spill the beans in a public hearing.

The events since then, however — most notably Attorney General William Barr’s handling of the matter — have conspired to create a situation in which Mueller could shed real light on some very important matters. And he could do it without even stepping outside his mandate.

Here are a few things he could speak to that could change this debate.

Addressing the confusion over his obstruction decision

Perhaps the most misleading narrative to come out of Barr’s previews of the report has been that it proved there was “no obstruction.” Barr initially said merely that Mueller simply hadn’t reached such a conclusion, but didn’t explain why. It turned out Mueller decided it wasn’t his place to do so because of Justice Department policy against indicting a sitting president. Even if the evidence was there, it seemed, he wouldn’t accuse.

But even as we now have that reasoning in black and white, misinformation and questions persist. Barr has said repeatedly that Mueller assured him he wasn’t saying that he would accuse Trump of obstruction but for that policy. But that makes it sound as if Mueller was conflicted about the evidence, when in actuality Mueller may have simply decided it wasn’t his place to make such a decision. (It would be kind of odd for Mueller to punt on that question but then just allow his actual conclusion to leak out through Barr.)

Former Mueller colleague Glenn Kirschner has said Mueller might respond to Barr’s mischaracterizations by just coming out and saying it: that Trump obstructed justice. I’m still dubious he’d go that far.

But even if he doesn’t, he could make it pretty clear how he feels about how Barr has handled explaining that decision. He could reinforce that he didn’t accuse Trump not because of the evidence, but because of the policy. And he could even talk through how compelling the evidence is on each of the 10 events in the obstruction probe - including the five for which he concluded that the evidence satisfied the three criteria for proving a crime.

How he feels about Barr’s conclusion

Even in arguing that he couldn’t accuse Trump of a crime, Mueller said he would “exonerate” Trump if he felt it was warranted. He said he could not do that. But then Barr and Deputy Attorney General Rod Rosenstein stepped in and effectively made that decision for him, saying the evidence wasn’t there on obstruction.

We’ve known about that disconnect for a while, but we don’t know how Mueller felt about it. Barr even said in his testimony last week that if Mueller wasn’t going to make a prosecutive decision, he should have shuttered his investigation. They seem to disagree on whether this is the Justice Department’s place.

Given that Barr has now criticized Mueller’s non-decision, it would seem fair game for Mueller to respond to that and lay out why he disagrees. And in doing so, he could indicate how he feels about how Barr and Rosenstein reached their decision.

How upset with Barr he really was

In his testimony, Barr was anxious to pretend that Mueller’s concerns were mostly with how his report was being characterized in the media. He made it sound as if maybe Mueller wasn’t even upset with Barr. But a March 24 letter, in which Mueller rebuked Barr’s handling of the matter, and Barr’s later testimony revealed the truth: Mueller was upset, and his decision to put his concerns in writing seems to back that up.

But just how upset? Was this a mere disagreement about how things were being handled at the time? Or does Mueller think it has paved the way for continued misinformation about his investigation and his conclusions.

Given that he expressed concern about such misinformation in the letter, it would seem he could expand upon whether that misinformation continues to carry the day — and perhaps even how much he continues to hold Barr personally responsible. It’s likely that Mueller would be diplomatic, but as with his letter, his words could allow us to pretty easily read between the lines.

“No collusion” vs. no conspiracy

The focus on the obstruction disagreements has obscured another very valid issue: whether it was OK for the Trump campaign to welcome Russia's help.

Mueller says at the start of Volume I that nothing the Trump campaign did rose to the level of a crime, but he also made clear he was evaluating the narrow legal concept of “conspiracy” rather than the broader and nonlegal concept of collusion.

With the right line of questioning, Democrats on the committee could really dig into how enthusiastic the Trump campaign was to accept this help. Mueller needn’t even step outside what’s in the report, but instead focus on the actual evidence and reinforce the distinction between conspiracy and collusion. This might not be getting as much attention as the obstruction portion, but it’s ripe for Mueller raising it if he wants.

Aaron Blake is a senior political reporter writing for The Fix. A Minnesota native, he has also written about politics for the Minneapolis Star Tribune and the Hill newspaper.