Washington (CNN) Constitutional law expert Jonathan Turley says his testimony Wednesday against impeaching President Donald Trump has led to a barrage of threats against him.

Turley, a law professor with nearly 30 years of teaching experience under his belt at George Washington University, was one of four scholars called before the House Judiciary Committee to provide insight into the complicated and rare constitutional maneuvering.

Turley was called by the committee's Republicans. The other three witnesses -- all called by the committee's Democrats -- spoke out in favor of Trump's impeachment while Turley, who was also an expert witness during Bill Clinton's impeachment proceedings, leaned on his previous testimony in the 1990s to explain his opposition to it.

"Before I finished my testimony, my home and office were inundated with threatening messages and demands that I be fired from George Washington University for arguing that, while a case for impeachment can be made, it has not been made on this record," Turley wrote in an op-ed published Thursday in The Hill.

Impeachment case 'has left doubts in the minds of people'

In his opening statement , Turley referred to his previous testimony, saying he "never thought that I would have to appear a second time to address the same question with regard to another sitting president."

"Yet, here we are," he added.

He continued: "Today, my only concern is the integrity and coherence of the constitutional standard and process of impeachment. President Trump will not be our last president and what we leave in the wake of this scandal will shape our democracy for generations to come. I am concerned about lowering impeachment standards to fit a paucity of evidence and an abundance of anger."

During his testimony, Turley consistently pointed to his worries about the speed and scope of the impeachment inquiry, saying, "Fast and narrow is not a good recipe." He also argued that the record against Trump is "one of the thinnest records ever to go forward on impeachment" and that it "has left doubts in the minds of people" about what happened.

"There's a difference between requesting investigations and a quid pro quo. You need to stick the landing on the quid pro quo," he said, referring to some Democrats' charge that Trump withheld military aid to Ukraine in exchange for the country announcing investigations that would benefit him politically. "I agree with my colleagues. We've all read the record and I've just come to a different conclusion. I don't see proof of a quid pro quo no matter what my presumptions, assumptions or bias might be."

But Turley's involvement in Clinton's impeachment proceedings may have weakened his testimony, as his comments in 1998 about the danger of not impeaching a president because it cedes too much power to the presidency did not square well Wednesday, when he seemed to take the opposite view: that impeachment itself created the bad precedent.

Turley did not respond on Thursday to questions from CNN about his testimony, instead referring to a blog post that dismissed reports saying his Clinton and Trump testimonies contradict one another.

"In fact, the testimony is so close that I could be charged more with self-plagiarism than self-contradiction," he wrote.

In a separate blog post , Turley said his testimony was meant to encourage additional investigation, not to stop the inquiry altogether.

"It is an ironic moment. President Donald Trump wants a fast impeachment. I do not blame him," Turley wrote. "He, and I believe the Democratic leadership, he knows that this will easily collapse. I encouraged that the case be strengthened to address deficiencies.

"I do have an opinion and offered an extensive analysis to support it. That is my opinion with which many can honest disagree," he added. "However, these attacks are more evidence of the 'passions' that have out-stripped the proof in this controversy."

A constitutional scholar

Turley's 53-page written testimony was filled with analysis on the impeachment proceedings against Presidents Andrew Johnson and Richard Nixon, as well as Clinton, demonstrating his academic prowess on the issue with its more than 100 citations.

The lengthy piece of writing comes as no surprise considering Turley, who has worked as a legal analyst for NBC and CBS, has authored more than 400 articles on legal and policy issues, according to his biography. During his testimony, he said he's represented a number of notable clients, including former US attorneys general "during the Clinton impeachment litigation over privilege disputes triggered by the investigation of Independent Counsel Ken Starr."

Additionally, Turley served in 2010 as the lead defense counsel for former federal Judge G. Thomas Porteous, whose Senate trial ended in his removal from the bench. He noted that the House manager in that case was California Democratic Rep. Adam Schiff, who is now leading the impeachment charge against Trump as chairman of the House Intelligence Committee.

And while he opposes the impeachment proceeding, Turley was also clear during the hearing where his positions on both the President and the executive branch stand, saying he's "not a supporter" of Trump and that he "voted against him in 2016."

He continued: "I tend to favor Congress in disputes with the Executive Branch and I have been critical of the sweeping claims of presidential power and privileges made by modern Administrations."

Turley, who first began at George Washington University in 1990, "and in 1998, became the youngest chaired professor in the school's history," holds a law degree from Northwestern University and completed his undergraduate studies at the University of Chicago, according to his biography.

George Washington University did not immediately comment for this report.