The former FBI chief is expected to dispute Trump's claim that Comey told him multiple times that he was not under investigation.
The former Federal Bureau of Investigation chief is expected to dispute Trump's claim that Comey told him multiple times that he was not under investigation, CNN reported, citing sources familiar with Comey's thinking.
That potential bombshell testimony -- in which Comey also may address whether Trump urged him to halt or ease up on an investigation into former national security advisor Michael Flynn and his ties to Russia -- comes Thursday before the Senate Intelligence Committee.
But on Wednesday the spotlight will be on three intelligence chiefs -- Director of National Intelligence Dan Coats, National Security Agency head Mike Rogers and interim FBI Director Andrew McCabe.
Members of the intelligence panel no doubt will question Coats, a former senator, about a Tuesday night Washington Post report.
Citing US officials, it said Coats had told associates that Trump asked him whether he could intervene with Comey to get the FBI to ease its probe of Flynn.
Coats reportedly discussed that March conversation with other officials and decided that interceding with Comey, as Trump had suggested, would be inappropriate.
Rogers is also known to have had discussions with Trump about the Russia investigations, and top Senate Democrat Chuck Schumer said he looked forward to public, unclassified testimony Wednesday from Coats and Rogers.
"Classification shouldn't be used as an excuse not to make things public that the public needs to hear," Schumer told reporters Tuesday.
"And the conversations that the president had with each of them about investigations are not classified in any way."
Adding to the drama, a top secret NSA report leaked to online news outlet The Intercept shows that hackers from Russian military intelligence repeatedly tried to break into US voting systems before last year's presidential election.
Keen to crack down on leaks, the Trump administration quickly announced the arrest of a 25-year-old intelligence contractor on charges she violated the espionage act.
The interactions between Trump and his now-dismissed FBI chief will likely dominate Comey's Senate appearance Thursday.
Observers will be closely watching to see whether the president, whose social media habits have been criticized even by his political allies, decides to respond via Twitter as Comey testifies.
No definitive evidence of collusion between Trump's campaign and Russia has come to light.
But the allegations have drawn comparison to the 1970s Watergate scandal that brought down president Richard Nixon.
Trump, a ratings-obsessed former reality television star, may not appreciate the worldwide attention paid to Comey's public testimony, which is being touted as the height of political drama.
Trump weighed in Tuesday, even as the revelations could spell more bad news for his troubled administration.
"I wish him luck," Trump said of Comey during a White House meeting with Republican congressional leaders.
With Capitol Hill abuzz, at least three Washington bars were set to open their doors before the Thursday hearing's 10:00 am (1400 GMT) start.
"Don't you like the suspense?" Senate Republican Marco Rubio, who is on the intelligence panel, asked a scrum of reporters.
Comey's testimony will be his first public remarks since Trump abruptly fired him on May 9.
Comey is said to have written detailed notes about conversations he had with Trump, which reportedly document the president's efforts to scuttle the FBI's Flynn probe.
A direct request from Trump during a February 14 meeting left Comey so unnerved, the New York Times reported Tuesday, citing current and former officials, that on the following day he told Attorney General Jeff Sessions that he did not want to be left alone again with the president.
Any confirmation that Trump tried to press Comey would open the president to damaging allegations that he attempted to obstruct an ongoing FBI investigation -- which several Democrats have warned would propel the crisis into Watergate-like territory.