Comey told the intelligence committee in a closed session that Sessions may have had a third, undisclosed interaction with Russia's ambassador to the USA, according to people familiar with the briefing.
Though we may not learn a lot about the Russian Federation investigation, we will be able to better assess Sessions' survival by the end of the day. In addition to his testimony, the fate of Special Counsel Robert Mueller is also in the air.
The Justice Department has said that while Sessions was there, for a speech by Trump, there were no meetings or private encounters.
There is a way that Sessions could skirt around having to directly answer these questions, however. Does he simply want to get his side of the story out following Comey's testimony? If you want to come to the Judiciary Committee and testify under oath, we'll put Comey right by you.
Sessions' appearance is not without risk. Feinstein said she was especially concerned after National Intelligence Director Dan Coats and National Security Agency Director Michael Rogers refused to answer questions from the intelligence committee about possible undue influence by Trump. Sessions has insisted his meetings with the Russian ambassador were a normal part of his Senate duties.
"You may be the first president in history to go down because you can't stop inappropriately talking about an investigation that if you just were quiet, would clear you", Graham said on CBS.
"I am not sure he is the best performer under questioning from fellow senators. No more game playing", Schumer said.
Sessions is vulnerable in at least three areas. And if he disagrees with Comey and explains in a public hearing the reasons for his recusal, it will likely lead to more questions about why he didn't also recuse himself from the firing of Comey.
"I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct", Comey testified. Multiple sources confirmed that the president was disappointed with the attorney general's recusal - a decision Trump only learned about minutes before it was announced in March. DOJ says the attorney general reminded Comey about the DOJ policy.
Trump was never happy with Sessions' March 2 decision to recuse himself from the Russian Federation probe.
The Comey hearing certainly lived up to the hype.
Sessions is likely to be asked what he and Russian Ambassador Sergey Kislyak discussed, and whether there are any other meetings he hasn't disclosed.
That remark came after revelations that Sessions had met with Russia's ambassador to the USA last year, despite testifying under oath during a confirmation hearing that he "did not have communications with the Russians".
When it later emerged he had in fact had several meetings with Kislyak, some Democrats accused him of lying to Congress and demanded his resignation.
On Monday, White House press secretary Sean Spicer said he wasn't sure whether Sessions would invoke executive privilege to limit the range of topics covered in the hearing. What can you tell us about this? "What it is not to be used for is as some kind of tool to cover up conduct that is improper or illegal perhaps", Moore said.
Tuesday's hearing may also throw new light on the awkward state of the relationship between Trump and Sessions.
In March, Sessions recused himself from any investigations into Russia's actions in the 2016 campaign.
The judge's decision issued Monday afternoon also gave the Justice Department and the FBI one month to search for any records of White House chief of staff Reince Priebus' reported outreach to the FBI requesting the bureau refute reports of communications between Russian officials and members of the Trump campaign.