Trump has challenged that assessment and has yet to say whether a full briefing with intelligence officials last week did anything to sway him.
"We are not here to relitigate the results of the election", Senator Mark Warner (D-Virginia), the ranking member on the committee, said in his opening remarks.
An unclassified version also was made public, outlining the agencies' claims of Russian actions created to undermine the election and help Trump by hurting Clinton.
Comey said that the agency's requests to examine the servers of the Democratic National Committee were denied - disputing a report this month that the Federal Bureau of Investigation never asked for access.
"If I were Trump, not sure why I'd take questions at a press conference tomorrow", tweeted Ian Bremmer, the author and president of the Eurasia Group, a political consulting firm.
The CNN reported earlier on Wednesday, citing US officials with direct knowledge of the briefings, that American intelligence chiefs briefed Trump last Friday about the alleged evidence that Russian operatives had comprising personal and financial information about the US President-elect.
Details of the reports began circulating in the autumn and were widely known among journalists and politicians in Washington.
On Wednesday, a spokesman for president Vladimir V Putin of Russian Federation dismissed the allegations.
Trump postponed a press conference originally set for December 15 to reveal how he would avoid potential conflicts of interest between his duties as president and his worldwide businesses.
The firm was paid for its work first by Mr Trump's Republican rivals and later by supporters of Mrs. Clinton. First, it a four-byline story from CNN: "Intel chiefs presented Trump with claims of Russian efforts to compromise him", from Evan Perez, Jim Sciutto, Jake Tapper, and Carl Bernstein.
Mr Trump never completed any major deals in Russian Federation, though he discussed them for years.
"Yes, there is credence possibly given to this, because the former British intelligence officer who is said to have put this together is considered credible, is considered to have very good relations with Russian intelligence". The videos were supposedly prepared as "kompromat", or compromising material, with the possible goal of blackmailing Mr. Trump in the future.
The series of events had some journalists concerned that the scheduled press conference might also be delayed following the latest reports on Trump and Russian Federation. We have been chasing specific claims in this document for weeks, and will continue to.