Among other things, Comey will almost certainly address explosive but disputed reports over whether Trump demanded loyalty from him during a private dinner in January, as Comey's associates have claimed.
Ron Wyden, a hard-charging member of the Senate Intelligence Committee, intends to use this moment to press Comey to detail what exactly happened in this exchange, a spokesman for the Oregon Democrat says.
"If and when Comey testifies, Senator Wyden will ask him if Donald Trump demanded a loyalty pledge," Wyden spokesman Keith Chu told me.
If Comey asserts in public that Trump did demand loyalty from him - which is plausible - consider what could happen then.
Trump responded to initial reports of that demand with a threatening tweet that implied Trump may have been taping private conversations. If Comey goes public, the pressure on the White House to release these tapes - or admit they don't exist - should intensify.
Republican lawmakers - who already expressed discomfort with the firing and with Trump's threat - will now be expected to comment about Comey's on-the-record assertion that the President demanded a loyalty pledge from him.
Some legal experts have suggested such a loyalty demand could constitute obstruction of justice.
It isn't just that FBI directors (who serve insulating 10-year terms) aren't supposed to be political loyalists.
It's also that Trump would have demanded loyalty from the man overseeing the FBI probe into his own campaign, even as that man (Comey) knew full well that Trump has the power to fire him, which Trump has now exercised, explicitly because of Comey's handling of the investigation.
This undermines basic norms dictating a clear separation between the White House and law enforcement and raises doubts as to whether the FBI's investigation can proceed free of political interference.
A serious probe of this whole affair by an independent commission or similar would look not just at possible Russia-Trump campaign collusion.
It would also seek to establish whether Trump demanded Comey's loyalty. It would seek a full accounting of Trump's decision to fire Comey, and what sort of role Attorney-General Jeff Sessions and his deputy, Rod Rosenstein, played in carrying that out.
That latter question matters, since Sessions was supposed to recuse himself from the investigation, and any role Rosenstein played in the political hatcheting of Comey could compromise his role in overseeing the continuing FBI probe.
But some GOP leaders continue to resist this full accounting.
The other day, House Speaker Paul Ryan responded to the Comey firing by claiming: "I'm focusing on what's in my control, and that is what is Congress doing to solve people's problems".
As Brian Beutler aptly points out, by saying the Comey firing is not in "his control," Ryan is actually "denuding himself" of the congressional GOP's oversight power and responsibilities:
What Ryan has done is surrender his own fundamental powers to Trump, knowing that people he likes and respects are telling reporters that Trump's presence in the White House terrifies them.
Indeed, Trump may take this as a sign that Republicans will not exercise any meaningful oversight even if the revelations grow worse.
The larger pattern here is crucial to appreciate.
Trump is constantly trying to probe what abuses our institutions will let him get away with.
That's whether it's:
- the in-your-face use of important diplomatic business to promote Mar-a-Lago and steer cash into his own pockets;
- the refusal to release his tax returns while advancing a tax plan that could deliver him and his family an enormous windfall;
- the constant lies about millions voting illegally in our election, which undermine public confidence in our democracy, and which he will now try to legitimise with a "voter fraud" commission;
- and, now, the firing of Comey.
Republicans have tolerated (or even embraced) all of these things, and Trump would be reasonable in concluding that they have no intention of ever mounting a check on his power, no matter what he does.
The question is whether a dramatic moment from Comey - in which he asserts that the President did, in fact, demand his loyalty - is enough to change this.
