In advance, the Intelligence Committee released a seven-page statement yesterday that serves as Comey's opening remarks (link here). That will be followed by what is expected to be lengthy questioning by senators on the committee.
Trump apologists have gleefully -- both foolishly and wrongfully -- announced that the statement vindicates Trump. It most certainly does not.
More often than not, the FBI does not open an investigation into a specific individual but instead into a possible criminal enterprise without naming specific targets. Then they go where the investigation takes them, and if any wrongdoing is uncovered, individuals can be charged.
For instance, Martha Stewart was never investigated by federal authorities. They never opened an investigation into her purchase or sale of particular securities.
Instead, they investigated the purchase and sale of a company's securities for possible insider trading. As part of that investigation, they learned that Stewart and others may have broken the law, and those people were charged accordingly.
Similarly, Richard Nixon was never the specific target of a federal investigation. The Watergate burglary and various potentially criminal enterprises were investigated and then certain people were charged. Nixon only escaped prosecution because he had a blanket pardon by President Gerald Ford.
So celebrating the fact that Trump was not the target of an investigation means nothing. His campaign and possible criminal activity is being investigated, and that may lead to criminal charges against Trump and/or persons close to him and connected with his campaign.
Today's hearing may reveal some bombshells, but it's quite possible that it might not. Secret details of the ongoing investigation will not be revealed.
In Watergate, the major part of the investigation was entirely invisible to the public and involved the FBI, federal prosecutors, and grand juries. Similarly, in the current Trump/Russia scandal, the FBI, federal prosecutors, and at least one grand jury are doing all the heavy lifting and that will remain secret until criminal charges, if any, are filed and people are taken into custody.
One important factor to watch for today, however, will be how vigorous Republican senators are in questioning Comey. Do they seem to be protecting Trump or are they being forthright and even aggressive in pursuing the truth?
If it is the latter, then that means Trump's support in Congress is a lot weaker than any of us may realize. If so, that will be very, very bad news for him.