The FBI has been accused of having a "double standard" in releasing potentially damaging material about Hillary Clinton while sitting on "explosive information" about Donald Trump.
US Senate Democratic leader Harry Reid wrote to FBI director James Comey about his decision on Friday to publicise the agency's discovery of new emails related to Hillary Clinton's private server.
"The double standard established by your actions is clear," he wrote in the letter.
"In my communications with you and other top officials in the national security community, it has become clear that you possess explosive information about close ties and co-ordination between Donald Trump, his top advisers, and the Russian government - a foreign interest openly hostile to the United States, which Trump praises at every opportunity.
"The public has a right to know this information. I wrote to you months ago calling for this information to be released to the public.
"There is no danger to American interests from releasing it. And yet, you continue to resist calls to inform the public of this critical information.
"By contrast, as soon as you came into possession of the slightest innuendo related to Secretary Clinton, you rushed to publicise it in the most negative light possible."
Clinton has been feeling the heat after Comey wrote to members of Congress to inform them the FBI had discovered new emails during an unrelated investigation.
The FBI is investigating whether there is classified information in the new emails, which were uncovered during the sexting investigation of Anthony Weiner, who is the estranged husband of Clinton aide Huma Abedin.
Reid said he believed Comey's actions in recent months demonstrated a disturbing double standard for the treatment of sensitive information, "what appears to be a clear intent to aid one political part over another".
He said Comey's actions may violate the Hatch Act, which bars FBI officials from using their official authority to influence an election.
"Through your partisan actions, you may have broken the law."
Comey's disclosure of the email discovery in a letter to Congress on Friday has thrown a spanner into the final days of the race between Clinton and Republican Donald Trump.
The Democratic candidate had opened a recent lead over Trump in national polls, but it had been narrowing even before the email controversy resurfaced.
The unexpected turn in the email controversy shook the financial markets' conviction of a Clinton victory in the election and the US dollar slipped against major currencies in early Asian trading on Monday.
FBI investigators have secured a warrant to examine the newly discovered emails to see if they are relevant to its probe of the private server used for government work by Clinton while she was secretary of state from 2009 to 2013.
Comey came under heavy pressure from Democrats on Sunday to quickly provide details of the emails, as Clinton allies worried the prolonged controversy could extend beyond the November 8 election and cast a shadow over a Clinton transition if she wins the White House.