Hillary Clinton isn't immune from congressional impeachment proceedings just because she's out of office – and the Democrat could be subject to an impeachment inquiry even while she campaigns for the White House, a leading open government expert tells DailyMail.com.

Tom Fitton, president of the conservative open government group Judicial Watch, made the startling disclosure even as a leading House committee chairman have written the FBI seeking an investigation into whether Clinton committed perjury during testimony about her emails before a House Benghazi committee.

'One of the dirty little secrets in Washington is that Mrs. Clinton can be impeached now,' said Fitton in an interview. 'A federal official leaving office does not end congressional jurisdiction over impeachment for misconduct in that office.

'And if she was convicted by the Senate the sanction could be ineligibility for future office.'

Fitton is author of a new book, 'Clean House,' which is subtitled, 'Exposing our government's secrets and lies.'

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Former secretary of state Hillary Clinton, pictured here testifying before a House special Benghazi committee in 2015, could be impeached even though she has left office

A central premise is that the administration must stop concealing information and Congress must stop holding back its own powers to force more disclosure.

'They need to talk about proactively using the contempt power more directly – not relying on the justice department to put people in contempt to enforce contempt powers on their own. And to use impeachment a whole lot more,' he said.

But Fitton said House GOP lawmakers – who got burned during the Bill Clinton impeachment in the 1990s – are too reluctant to deploy their probative powers.

'The Congress thinks that Mrs. Clinton lied to them. You have the mishandling of classified info that was quite evident and other violations of the law including federal records laws with the associated graft of the Clinton Foundation. There’s plenty to begin at least an impeachment inquiry in the House,' he said.

Conservative watchdog group Judicial Watch has filed more than 3,000 lawsuits during the Obama administration to obtain information

The group's litigation is credited with forcing the eventual disclosure of Hillary Clinton's undisclosed private email server

The group's constant litigation has pestered administration officials, including longtime Hillary Clinton aide Huma Abedin. Judicial Watch president Tom Fitton predicts another 10,000 Abedin emails will be forthcoming.

Judicial Watch president Tom Fitton says one of the 'dirty little secrets' in D.C. is that Hillary Clinton can be impeached now

'Once you mention that to members on the Hill, they fall on the floor and start shaking. That is as welcome an idea as a special counsel is with [attorney general] Loretta lynch.'

Republican presidential candidate Donald Trump has called for a special counsel to investigate Clinton 'immediately.'

'Congress is pretending the Justice Department has to do all the work. That’s not true. Why doesn’t Congress hold Hillary Clinton in contempt? Why doesn’t Congress initiate an impeachment inquiry?' he asked.

The book traces the litigation, original begun seeking documents related to the attack on the U.S. facility in Benghazi, that ultimately resulted in disclosures about Clinton's undisclosed private email server.

'I have no doubt that our litigation forced their hand,' said Fitton, who noted the State Department's cryptic references to additional materials that he speculated preceded an agency leak to the New York Times.

The group has mostly hounded Democrats, but also pursued an investigation of fundraising practices of former House Majority Whip Tom DeLay of Texas

Ousted IRS official Lois Lerner was in the hot seat after agency documents revealed apparent targeting of conservative groups

The group also sought documents related to the botched Obamacare rollout

Judicial Watch got its start going after the Bill Clinton administration. It continues to seek documents relating to apparent efforts to boost donors to the Clinton Foundation

'The pressure from the investigations forced them to disclose it. It wasn’t voluntary disclosure. And the first inkling of it was in our lawsuit,' he said.

He said the agency was still 'protecting Clinton's email,' and that the process would proceed well into the next administration.

'I think there are going to be some uncomfortable decisions for the next administration. No matter who is elected there is going to be intense pressure to get a criminal investigation going. Just because Mrs. Clinton may win doesn’t mean there won’t be a criminal investigation. In fact, it may be more likely,' he said.

The lawsuit that ultimately led to disclosures about the emails related to the development of 'talking points' about the Benghazi attack.

After the State Department at first said it could find no more documents requested in a FOIA lawsuit, it revealed in February 2014, 'In the course of preparing additional information to provide to Plaintiff for purposes of settlement discussions, Defendant has discovered that additional searches for documents potentially responsive to the FOIA must be conducted.'

Revelations about Clinton's email began with a probe of the attack on Benghazi

The revelation was a cue that more emails had been uncovered.

According to the book, 'Now we know the Obama administration consciously refused to give up key information about Hillary Clinton’s email in 2014. It covered up this email both from the court and Judicial Watch. The cover-up provided Hillary Clinton enough time to hide potentially thousands of government records. One aim of our court- ordered discovery will be to get to the bottom of this cover-up.'

The group continues to battle the State Department in court – pushing to expedite release of 15,000 Clinton emails before the elections.

Fitton revealed that it isn't just Clinton who will have to contend with the drip-drip-drip of emails after the elections, but longtime aide Huma Abedin, the subject of separate litigation by the group.

Fitton said the agency's document production has only reached mid-2009 – which does not yet include the time in 2011 and 2012 when Abedin was working both as a State Department employee and a consultant for the Clinton Foundation.

'Can you imagine what those emails are like?' he said. 'I’d say there’s probably well over 10,000 more pages of documents they haven’t given us yet.'

The book traces the flood of litigation that has given the Obama administration headaches, including the IRS targeting scandal, Benghazi, Clinton emails, 'Fast and Furious,' Obamacare, and other issues.

In a demonstration of how the group's litigation can annoy the administration, the 'Clean House' book, published by Simon and Schuster's Threshold imprint, goes into extensive detail about Abedin's obtaining status as a 'Special Government Employee' who got to do outside work while at State.

It notes that she became a senior government employee without having filed her required financial disclosure forms and failing to report assets of her husband, former Rep. Anthony Weiner, who Abedin announced last weekend she is separating from after new details of his electronic sexting.

It quotes a memo from an agency bureaucrat telling Abedin: 'I have your termination OGE-278 report and the financial disclosure report for the Senior Adviser position. While reviewing your termination OFE-278, I noticed your spouse had several assets that aren’t reported on your report. Can you kindly provide an end-of-year summary statement so that I can update your report immediately?'

The group also sued to find out whether Clinton, Abedin, or chief of staff Cheryl Mills had attended training on handling of 'Classified and Sensitive but Unclassified Information.'

Some of the emails obtained have been fodder on the campaign trail, helping fuel Donald Trump's attack on Clinton's 'stamina.'

An email referenced in the book is a 2013 missive from Abedin to Clinton aide Monica Hanley: 'Have you been going over her calls with her? So she knows that Singh (Indian Prime Minister Manmohan Singh) is at 8?' Abedin asks.

''She was in bed for a nap by the time I heard that she had an 8am call. Will go over with her,' Hanley replies, to which Abedin responds: 'Very imp to do that. She’s often confused.

The group has 20 active lawsuits relating to the Clinton email system alone, it says. These included a review of conflict of interest reviews for Bill Clinton speeches – which included 223 events with 311 sponsors.