The FBI director said it made him "mildly nauseous" to think the FBI may have had some impact on the election, but that he was forced to choose between saying something or concealing information.
Speaking at a Senate Judiciary Committee oversight hearing on Capitol Hill, Comey said he struggled with the initial choice and felt dismayed if his actions had any effect on the outcome of the race.
"It wasn't a ideal campaign, but I was on the way to winning until a combination of Comey's letter and Russian WikiLeaks", the Democrat said of her loss to Mr Donald Trump. "The president won 306 electoral votes and I think there's been plenty of analysis on the election and where people chose to spend their time and their resources and their messaging and I think it's somewhat sad that we're still debating why the president won in the fashion that he did".
Comey told the senate committee the Federal Bureau of Investigation reopened the probe after agents discovered additional classified emails forwarded by Clinton aide Huma Abedin to her husband, who was not authorized to view them.
FBI Director James Comey says he believed it would have been "catastrophic" to keep Congress in the dark about new developments in the Hillary Clinton email investigation that emerged 11 days before Election Day. Not doing so amounted to "concealment", which he said would have been "catastrophic".
Persistent questions from senators, and Comey's testimony, made clear that the FBI director's decisions of last summer and fall involving both the Trump and Clinton campaigns continue to roil national politics and produce lingering second-guessing about whether the investigations were handled evenly.
The line of questioning is similar to questions House Republicans pressed Comey on during the House Intelligence Committee hearing in March.
Trump tweeted later Tuesday night that Comey "was the best thing that ever happened to Hillary Clinton in that he gave her a free pass for many bad deeds".
Arsenal and Manchester City fans videoed fighting outside Wembley
Alexis Sanchez scored an extra-time victor as the Gunners came from behind to win 2-1 and reach a record 20th final - and an eighth under Wenger.
"We need to know whether there was anything improper going on between the Trump campaign and the Russians, or if these allegations are just a partisan smear campaign that manipulated our government into chasing conspiracy theories", Grassley said.
Earlier in the day, Clinton discussed her 2016 election loss with CNN's Christiane Amanpour at a Women for Women International event in NY.
He denied claims by Democrat senators that he had displayed a double standard on treatment of the two candidates and would add nothing more on the Trump investigation.
White House spokesman Sean Spicer ripped Hillary Clinton's claim that she would have won the 2016 election if it had been held on October 27, saying she did not get to determine election dates and that it was "somewhat sad" to still be litigating why Donald Trump won.
WikiLeaks previous year published emails from Democratic email accounts that U.S. intelligence officials believe were hacked by Russian Federation.
Comey said on Wednesday the probe into whether Russian hacking activities and American citizens was a classified investigation in its early stages - far different from the nearly completed email probe.
FBI Director James Comey said Wednesday that his bureau is looking into whether there were possible internal leaks to Rudy Giuliani, promising "severe consequences" if there were. "Just push it out in order to damage".