FILE PHOTO: Former FBI Director James Comey testifies before a Senate Intelligence Committee hearing on Russia's alleged interference in the 2016 U.S. presidential election on Capitol Hill in Washington, U.S., June 8, 2017. REUTERS/Jonathan Ernst

WASHINGTON (Reuters) - U.S. President Donald Trump, his White House staff and the Republican National Committee escalated their attacks on Monday against the former director of the Federal Bureau of Investigation who called Trump a "morally unfit" leader in an interview.

James Comey, who was fired by Trump last year amid an FBI investigation of possible ties between Russia's meddling in the U.S. election and Trump's 2016 campaign, made his comments in an interview with ABC News on Sunday, saying Trump may be vulnerable to blackmail by Russia.

Comey has written a book to be released on Tuesday that also is highly critical of Trump. During the weekend the president had tweeted that Comey was a "slimeball," "a leaker & liar" and a terrible FBI director. On Monday, he returned to Twitter to accuse Comey and "others" of committing "many crimes."

Trump provided no evidence of a crime and Comey has not been accused of wrongdoing by legal authorities. He may be a witness for any charges of obstruction of justice under consideration by U.S. Special Counsel Robert Mueller, who is investigating connections between Trump's campaign and Russia.

Both Moscow, Trump and the White House deny any wrongdoing.

Trump's latest Twitter broadside was part of a series of attacks by Republicans and the White House against Comey, who they have attempted to discredit and paint as a disgruntled employee.

"This is somebody who's giving a revisionist version of history ... The president is confounded that this person is always able to divert the spotlight to him," White House adviser Kellyanne Conway said in an interview with ABC News on Monday.

The Republican National Committee also created a website, https://lyincomey.com, using an insult first used in a tweet by Trump, to attack Comey.

"This is somebody who wants a payday," said Ronna McDaniel, the chairwoman of the Republican National Committee, told CNN.

"He has taken his private conversations with the president and monetized them. I think that is something that is discrediting and we're going to push back," McDaniel said.

Comey's book, "A Higher Loyalty: Truth, Lies and Leadership," has been on Amazon best-seller lists for the past four weeks and, as of Monday morning, is No. 20 on the retailer's top nonfiction sales.

(Reporting by Makini Brice; Editing by Bill Trott)