FBI Director James Comey said on Wednesday it made him "mildly nauseous" to think his announcement of the reopening of an investigation into Hillary Clinton's emails affected the 2016 presidential election, but he had no regrets and would make the same decision again.

Clinton said on Tuesday her election bid was derailed in part by Comey's announcement about the renewed probe of her use of a private email server while she was secretary of state.

In four hours of testimony before the Senate Judiciary Committee, Comey delivered his most impassioned defense yet of a decision that many Democrats believe cost them the White House: telling Congress 11 days before the Nov 8 election that the FBI had uncovered a new trove of Clinton-related emails.

Comey said he felt he had to speak out at that time because he had repeatedly told lawmakers the Clinton email probe was over. "To not speak about it would require an act of concealment in my view," Comey said. "Concealing, in my view, would be catastrophic."