Alan Dershowitz said the Democrats have exposed Robert Mueller to a grilling over claims the FBI spied on the Trump campaign in demanding his testimony next month.

The veteran lawyer told Fox News that Republicans would now be able to tear into the special counsel over the Foreign Intelligence Surveillance Act (FISA) application - a request by the FBI to monitor a member of the Trump campaign team in 2016.

Dershowitz told The Ingraham Angle: 'Those are the kind of questions that I think Republicans will be very well prepared to ask.

'Those are the kind of questions which are currently under investigation by the inspector general whose report we are waiting for.

'But those are not in any way precluded. So I think that they will regret having called him.'

President Donald Trump referred to the FISA warrants approved by the Obama Justice Department as 'bigger than WATERGATE' last month.

Alan Dershowitz said nothing would preclude Republicans from grilling special counsel Robert Mueller on subjects which could hurt the Democrat cause

Veteran lawyer Alan Dershowitz said Republicans would target allegations of Obama-era spying by the FBI on the Trump campaign

The president mocked The New York Times last month for 'finally' reporting on Obama-era efforts to allegedly spy on his campaign

Trump fired FBI director James Comey two years ago, who had led the Russian interference probe which was later taken up by Mueller.

Attorney General William Barr told Congress in April that 'spying on a political campaign is a big deal,' and added: 'I think spying did occur.'

The president referred to those comments last month, saying: 'The word "spying" has been used. He [Comey] probably was one of the people leading the effort on spying.'

Dershowitz suggests the Republicans will now have a chance to take aim at those Trump campaign 'spying' claims.

Special counsel Robert Mueller has agreed to testify publicly before Congress on July 17 after Democrats issued subpoenas to compel him to appear, the chairmen of two House committees announced Tuesday.

Republicans have slammed them for their continuing investigations of President Donald Trump since the lengthy Russian interference report was filed.

Doug Collins, the most senior Republican on the Judiciary Committee, said in a statement: 'I hope the special counsel’s testimony marks an end to the political gamesmanship that Judiciary Democrats have pursued at great cost to taxpayers.'

But Collins also welcomed Mueller's appearance, saying: 'May this testimony bring to House Democrats the closure that the rest of America has enjoyed for months, and may it enable them to return to the business of legislating.'

Meanwhile House Minority Leader Kevin McCarthy (R-Calif.) questioned the hearing: 'He said he didn't want to talk to us anymore, didn't he?'

Mueller's unusual back-to-back testimony in front of the House Judiciary and Intelligence committees is likely to be the most highly anticipated congressional hearing in years, particularly given Mueller's resolute silence throughout his two-year investigation into Russian contacts with President Donald Trump's campaign.

Mueller never engaged in public attacks from Trump, nor did he ever personally join his prosecutors in court or make announcements of criminal charges from the team.

President Donald Trump simply tweeted, 'Presidential Harassment!' after yesterday's announcement

His sole public statement came from the Justice Department podium last month as he announced his departure, when he sought to explain his decision to not indict Trump or to accuse him of criminal conduct.

He also told lawmakers that he never intends to say more than what he put in the 448-page report.

'We chose those words carefully and the work speaks for itself,' Mueller said May 29. 'I would not provide information beyond what is already public in any appearance before Congress.'

Those remarks did little to settle the demands for his testimony.

The two committees continued negotiations that had already been going on for weeks, saying they still wanted to hear from Mueller no matter how reluctant he was.

'When you accept the role of special counsel in one of the most significant investigations in modern history you're going to have to expect that you're going to be asked to come and testify before Congress,' House Intelligence Committee Chairman Adam Schiff (D-Calif.) told reporters shortly after the announcement.

Trump himself simply tweeted, 'Presidential Harassment!'

In the report issued in April, Mueller concluded there was not enough evidence to establish a conspiracy between Trump's presidential campaign and Russia, which was the original question that started the investigation.

Ranking Member of the House Judiciary Committee Doug Collins said: 'I hope the special counsel’s testimony marks an end to the political gamesmanship that Judiciary Democrats have pursued at great cost to taxpayers'

But he also said he could not exonerate Trump on obstruction of justice. The report examined several episodes in which Trump attempted to influence the investigation.

Democrats say it is now the job of Congress to assess the report's findings.

Lawmakers are likely to confront Mueller on why he did not come to a firm conclusion on obstruction of justice.

They are also likely to seek his reaction to a drumbeat of incessant criticism from the president and ask for his personal opinion about whether Trump would have been charged were he not the commander-in-chief.

Schiff and House Judiciary Committee Chairman Jerrold Nadler said they issued the subpoenas Tuesday, and Mueller agreed to testify pursuant to those subpoenas.

In a letter to Mueller accompanying the subpoenas, the committee chairmen said 'the American public deserves to hear directly from you about your investigation and conclusions.'

Schiff said there will be two hearings 'back to back,' one for each committee, and they will also meet with Mueller's staff in closed session afterward.

The Justice Department declined to comment.