CLEVELAND/GRAND RAPIDS, Michigan, Oct 31 (Reuters) - Democrats kept up their attacks on FBI Director James Comey on Monday, accusing him of a double standard after he revealed his agency’s probe into more material that might relate to presidential candidate Hillary Clinton’s use of a private email server.

Little is publicly known yet about the trove of emails being investigated, other than that they were found during an unrelated probe into the estranged husband of a top Clinton aide. Comey himself said in a brief letter about the issue to members of Congress on Friday that “we don’t know the significance of this newly discovered collection of emails.”

But Clinton’s Republican rival, Donald Trump, seized on the news of the probe to press his longstanding charge that Clinton lacks integrity, hoping he can make an improbable late comeback and win the Nov. 8 election.

Opinion polls have shown Clinton’s lead over Trump was narrowing slightly since early last week. It is not yet known if the email controversy will hurt her support. Millions of Americans have already cast their ballot in early voting.

The FBI spent a year investigating Clinton’s use of a private email server, instead of government systems, while she was secretary of state from 2009 to 2013. Comey concluded in July that whileClinton and her staff had been “extremely careless” in handling classified information there were no grounds for any charges.

But the issue resurfaced unexpectedly when Comey told Congress on Friday the agency was looking at new emails. Trump, a wealthy New York businessman, used the news to charge thatClinton represents a corrupt political system.

The Clinton campaign and its supporters furiously attacked Comey for releasing information that raised questions but provided no details so close to Election Day. Some party leaders said the agency was concealing damaging information about the Trump campaign.

The White House steered clear on Monday of direct criticism of Comey, who was appointed by Democratic President Barack Obama in 2013. Obama views the FBI head as a man of integrity and does not believe he is secretly trying to influence the outcome of the election, White House spokesman Josh Earnest told reporters.

However, he added that Obama believes that regarding the FBI it is important for norms and traditions about making information public to be followed.

Clinton was campaigning in Ohio on Monday and trying to move beyond the controversy, telling supporters to keep focused on winning the election.

“There is no case here,” she said at a rally at Kent State University. Earlier, she told a restaurant patron at a cafe in Cleveland that the email affair is, “Lots of noise. Lots of distractions. Throwing stuff at me. And we’ve just got to keep people up, moving and voting.”

‘EXPLOSIVE’

U.S. Senator Harry Reid, the Democratic leader in the Senate, accused Comey on Sunday of “a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another.”

He said, without providing evidence, that the FBI was keeping “explosive information” under wraps about ties between the Trump campaign and Russian officials.

Democratic Representative Elijah Cummings hammered this accusation on Monday, urging the FBI to release information on Trump and his advisers’ dealings with Russia.

An FBI spokeswoman said late on Sunday: “When we receive the letter it will be handled through our usual process in responding to members of Congress,” referring to Reid’s accusation.

The U.S. government has accused Russia of a campaign of cyber attacks targeting the Democratic Party that has led to the release of thousands of illegally obtained emails, revealing the sometimes unflattering inner workings of the party. The Kremlin has denied this. Trump has declined to implicate Russia in any wrongdoing.

Democrats have demanded that Comey and the FBI rapidly work to make public what they know about the new email trove. A source familiar with the matter said on Sunday that the FBI had secured a warrant to examine the emails.

The latest emails were discovered as part of a separate probe of former Democratic U.S. Representative Anthony Weiner, the estranged husband of Huma Abedin, who is Clinton’s closest aide and confidante, sources close to the FBI investigation have said. The FBI is investigating illicit text messages Weiner is alleged to have sent to a 15-year-old girl in North Carolina.

The Wall Street Journal reported that federal agents are preparing to spend weeks examining about 650,000 emails contained on Weiner’s laptop for possible links to Clinton’s email use. It is not known yet whether the FBI will release any information about its findings before Nov. 8.

Cummings said on Monday that the FBI’s vague statement on new emails related to the Clintoninvestigation made it impossible for Clinton to defend herself and “basically gave Donald Trump a softball to hit over the fence.”

On Sunday, Clinton campaign chairman John Podesta and campaign manager Robby Mook questioned the FBI head’s decision to notify Congress of the email review before he even knew whether they were significant or relevant.

Earnest said the White House had no independent knowledge of why Comey had sent the letter to Congress, but the White House was neither going to defend nor criticize the move.

“The president thinks very highly of Director Comey,” Earnest told a regular briefing. “The president doesn’t believe that he’s secretly strategizing to benefit one candidate or one political party. He’s in a tough spot.”

(Additional reporting by Doina Chiacu, Susan Heavey, Richard Cowan and Mark Hosenball in Washington; Writing by Frances Kerry; Editing by Alistair Bell)