Some legal experts said Comey's testimony could strengthen any impeachment case built on the obstruction of justice, but US markets shrugged off news from the hearings so far for a lack of any major disclosures.
Former FBI Director James Comey will testify on Thursday to a US Senate Intelligence Committee probing the Russian hack of last year's presidential election.
Comey's opening statements were released to the public on Wednesday evening. They suggest that Comey will tell Congress that President Donald Trump pressed him repeatedly to halt a probe into his ex-national security adviser Michael Flynn's ties with Russia and to declare publicly that Trump himself was not under investigation.
Comey's testimony is the most widely anticipated congressional hearing in years in the US.
The outcome could have significant repercussions for Trump's presidency as special counsel Robert Mueller and multiple congressional committees investigate whether Trump's campaign team colluded with Russia during the 2016 presidential election. The White House and Russia deny any collusion occurred.
The president on Wednesday was in Ohio, where he ignored reporters' questions about Comey.
TRT World's Azadeh Ansari reports from Washington.