After months of rumors and anonymously-sourced media reports, angry tweets and snarky cable news commentary, James Comey is finally under oath this morning and President Trump is very much on trial.

The Senate Intelligence Committee is set to spend all day listening to and questioning former FBI Director Comey, first in a public session that is being televised on major broadcast and cable channels and later behind closed doors. It is likely to be the most compelling political television seen in the U.S. since the Watergate hearings of the early 1970s.

Thanks to Wednesday’s release of his seven-page prepared statement, we already know that Comey will sketch out in finely-grained detail a series of meetings in which he says President Trump asked him to shut down an investigation of retired Lt. Gen. Michael Flynn’s connections to the Russian government.

Trump already has rebutted that allegation. Weeks ago in a press conference, the president denied that he had attempted, “in any way, shape, or form” to interfere with or end the Flynn investigation. How credibly Comey makes the case that Trump’s denial was a lie could determine the future of Trump’s presidency.

Comey is likely to field friendly questions today from Democratic senators and skeptical-to-hostile queries from Republicans. The White House pre-spun his testimony by spotlighting portions of Comey’s prepared statement in which he acknowledged repeatedly telling Trump that the president’s personal conduct was not the subject of any criminal investigation.

Maybe it wasn’t then, but it almost certainly is now or will be by day’s end. Comey is under oath today and legal experts already are suggesting his prepared remarks make a plausible case of obstruction of justice against the president. Congressional staffers are digging out and reviewing transcripts of the Nixon and Clinton impeachment hearings; some are drafting articles of impeachment.

Even if Comey’s testimony today somehow undercuts that case, a decidedly unlikely prospect given his decades of experience as a criminal investigator and witness, the gears of the legal system and congressional oversight of the executive branch will continue to turn. This really is just the beginning.