Richard Painter, a chief White House ethics lawyer to former Republican President George W Bush, on Saturday accused Comey of violating the 1939 Hatch Act when the FBI chief wrote Congress on Friday that more of the candidate's emails would be scrutinized.
On Sunday, US Senate Democratic Leader Harry Reid wrote to Comey to accuse him of partisan actions that may have broken the law.
The Hatch Act bars government employees from taking part in political activities that include soliciting or accepting donations from a political party and using official authority to interfere with the outcome of an election.
Stephen Vladeck, a professor at the University of Texas School of Law, said, "I would be surprised if this complaint leads to anything significant, but I would also be surprised if Comey's tenure as the FBI director will not be shortened."
"Even if he didn't violate the letter of the Hatch Act, he certainly violated the spirit of the Hatch Act, which should have prompted him to think twice before issuing that letter. Especially because the letter didn't say anything," he said.
Little is publicly known about the new email trove. Comey wrote Congress the FBI had yet to determine the significance.
Painter filed his complaint with the independent Office of Special Counsel (OSC) and the Office of Government Ethics. A spokesman for the OSC, which has a specialized Hatch Act investigative unit, declined to comment.
A Hatch Act investigation is an administrative, not a criminal, matter. Penalties may include removal from office, a demotion or temporary suspension, or a civil penalty no higher than $1000. In cases of high-level officials, the disciplinary decisions on OSC actions are made by the president.
On Monday, the White House said President Barack Obama believes Comey is a man of integrity and is not trying to influence the election by announcing the scrutiny of additional emails.
Jan Witold Baran, a lawyer at Wiley Rein in Washington, suggested it would be hard to construe what appeared to be Comey's engagement in normal government business as a violation of the Hatch Act.
But Kenneth Gross, former associate general counsel of the Federal Election Commission, said Comey's letter fell outside the FBI director's normal duties as there is no legal obligation to disclose details of an ongoing probe to Congress.
Increased discipline
Painter's complaint said that barring extraordinary circumstances, a public communication about a pending FBI investigation involving a candidate for public office so close to an election likely violated the act.
Comey said in an internal memo obtained by news media that even though it is not common to update Congress on the progress of investigations, he felt obligated because he had repeatedly said the probe was completed.
The OSC Hatch Act unit, which usually experiences a surge in complaints during election years, has disciplined 84 employees the last five years, almost a three-fold increase over the previous five years, an OSC 2015 report to Congress said.
Hatch Act violations usually relate to inappropriate electioneering. For example, the 2015 report cited a complaint against a US Agriculture Department official who allegedly asked two subordinate employees to donate to a political action committee supporting Obama's 2012 re-election.
In a 2016 letter, the OSC found that Secretary of Housing and Urban Development Julian Castro violated the act by advocating for and against presidential candidates during a media interview. Castro called it an "inadvertent error."
FBI not trying to influence vote: Obama
The White House on Monday opted for caution in reacting to FBI Director James Comey's bombshell decision to announce further investigation of Hillary Clinton's email days before the 2016 presidential election.
Spokesman Josh Earnest said he would "neither defend nor criticize" Comey, who has been blasted by Democrats since Friday.
Earnest said President Barack Obama considered Comey - a Republican appointee - to be "a man of integrity, a man of principle and he's a man of good character".
But he also pointed to norms that "limit public discussion of facts that are collected in the context" of ongoing investigations.
"The president believes that it's important for those norms and traditions and guidelines to be followed," the spokesman said.
Earnest said that Obama did not share the view of some Democrats who have suggested Comey was seeking to tilt the vote in favor of Republican Donald Trump through the new revelations.
"The president doesn't believe that director Comey is intentionally trying to influence the outcome of an election," Earnest said. "The president doesn't believe that he's secretly strategising to benefit one candidate or one political party."
Earnest did however acknowledge the intensity of criticism facing Comey, from all quarters.
"He's in a tough spot, and he's the one who will be in a position to defend his actions in the face of significant criticism from a variety of legal experts, including individuals who served in senior Department of Justice positions in administrations led by presidents in both parties."
"But I'll neither defend nor criticize what director Comey has decided to communicate to the public about this investigation," Earnest said.
Clinton says 'no case' in email FBI review
Hillary Clinton said Monday there was "no case" to be found in a federal review of her emails, which the FBI director announced last week, convulsing the US election campaign in its home stretch.
"I'm not making excuses. I've said it was a mistake and I regret it," she said Monday of her decision to use a private email server while secretary of state, which sparked a controversial FBI investigation.
FBI boss James Comey announced in a letter to lawmakers that his agents are reviewing a newly discovered trove of emails, resurrecting an issue Clinton had hoped was behind her.
US media says those emails were found on a laptop used by one of her closest aides, Huma Abedin, and Abedin's estranged husband who is being investigated for alleged sexual overtures to a 15-year-old girl.
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"Now they apparently want to look at emails of one of my staffers, and by all means, they should look at them," Clinton said.
"I am sure they'll reach the same conclusion they did when they looked at my email for the last year.
"There is no case here," she said to cheers and applause from hundreds of supporters at a rally in Kent, Ohio, a key battleground state in her race to defeat Donald Trump at the ballot box on November 8.
It was the first time Clinton directly addressed the email furor since Saturday, when she lashed out at Comey, calling his move "deeply troubling" while campaigning in another vital swing state, Florida.
Comey's announcement thrust back into the spotlight allegations that Clinton put the United States at risk by using the private email server.
Comey in July accused Clinton of being "extremely careless" but did not recommend that charges be brought against her, angering Republicans.
The Democratic nominee is still the overwhelming favorite to win the presidency, but polls have pointed to an increasingly tight race.
Clinton used much of her speech at Kent State University, 40 miles (60 kilometers) south of Cleveland to paint her Republican opponent as unfit for office and a threat to national security.
Trump holds the slenderest of leads in the state, where he is just one point ahead at 45.3 per cent to 44.3 per cent in a four-way race, according to the poll average from RealClearPolitics.
