Justice Department Inspector General Michael Horowitz said Wednesday that a senior prosecutor failed to convince him that the FBI's 2016 investigation of President Donald Trump's campaign was improperly opened, revealing new details about internal tension among senior officials over the politically explosive case.

At a Senate Judiciary Committee hearing, Horowitz was asked by the panel's senior Democrat, Sen. Dianne Feinstein of California, whether Attorney General William Barr or his hand-picked prosecutor on the issue, Connecticut U.S. Attorney John Durham, offered anything to change the inspector general's view that the FBI had a valid reason to open the probe in July 2016.

"No, we stand by our finding," said Horowitz, who said he met in November with Durham to discuss the findings in the inspector general's 434-page report released Monday.

When the report was released, Durham issued an unusual public statement saying he did not agree with Horowitz's conclusion about the opening of the investigation.

Horowitz told lawmakers that the disagreement stemmed from a difference of opinion about whether the FBI should have opened a preliminary investigation, which puts some limitations on the investigative steps that can be taken, or a full investigation.

The FBI opened a full investigation, based on a tip from the Australian government.

Durham "said during the meeting that the information from the friendly foreign government was in his view sufficient to support the preliminary investigation," Horowitz said.

Horowitz said he was "surprised" to see Durham issue a public statement disagreeing with that part of his report. Barr also disagrees with Horowitz on the issue, and in media interviews Tuesday accused the FBI of having acted in "bad faith" by pursuing the case.

Horowitz's testimony marked his first public pushback to Barr and Durham, and further revealed the depths of the disagreement among senior law enforcement officials about Horowitz's findings. Before the report was released publicly, The Washington Post reported that Barr disputed Horowitz's conclusion that the FBI had sufficient grounds to open the investigation.

Republicans on the committee castigated the FBI over its investigation into the Trump campaign, declaring that such a shoddy case should never be pursued again.

Sen. Lindsey Graham, R-S.C., the committee chairman and a frequent defender of the president, focused on Horowitz's finding of "serious performance failures" at the FBI.

Graham said politicians of both parties should be alarmed to see how the FBI launched an investigation into a political campaign, told no one inside the campaign of their concerns, and kept pursuing that investigation even after gathering a significant amount of exculpatory evidence.

"We can't write this off as being just about one man or one event. We've got to understand how off-the-rails the system got," Graham said. "I think Democrats and Republicans are willing to make sure this never happens again."

Invoking the secretive Foreign Intelligence Surveillance Court that approved surveillance as part of the investigation, Graham said he had "serious concerns" about whether the court can continue "unless there is fundamental reform."

Asked about the FBI's handling of the applications to the court, Horowitz said, "I think the activities we found here don't vindicate anybody who touched this FISA." FISA stands for the Foreign Intelligence Surveillance Act, the law that governs federal law enforcement's use of wiretaps in intelligence and terrorism cases. a

Horowitz noted his investigators found no "documentary or testimonial evidence that political bias or improper motivation influenced the FBI's decision to conduct these operations," but they did find failures up and down the chain of command, "including FBI senior officials who were briefed as the investigation progressed."

Republicans and Democrats have trumpeted the parts of the report that validate their wildly opposing views regarding the nature of the FBI's investigation, as either a baseless "witch hunt" or a fundamental defense of American democracy.

Graham attacked those who emphasized Horowitz's finding that the FBI had adequate cause to open the Russia case. Such a takeaway, he said, ignores that the inspector general also found serious wrongdoing in how the FBI conducted the investigation, particularly in its applications to a court to surreptitiously monitor former Trump campaign adviser Carter Page.

Graham said that he would assume, "for the sake of argument," the FBI had an adequate basis to open a counterintelligence investigation - though he noted the standard for doing so is low. But the other failures, he said, were more than a few modest mistakes.

"What happened here is not a few irregularities," Graham said. "What happened here is the system failed. The people at the highest levels of our government took the law into their own hands."

Feinstein defended the FBI as doing what it had to do to pursue disturbing allegations of election year wrongdoing.

"This was not a politically motivated investigation. There is no 'Deep State'," she said, referencing a pejorative used by Trump and his Republican allies to describe career government civil servants they perceive to be biased against the president. "The FBI's motivation was motivated by facts, not bias."

In contrast, Graham invoked J. Edgar Hoover, a former FBI director whose name has become synonymous with law enforcement abusing its power. He asserted that the FBI should have briefed Trump or other campaign officials when they suspected Russian involvement with the election effort but failed to do so, choosing instead to investigate. He read a series of anti-Trump texts from two officials involved in the case - agent Peter Strzok and lawyer Lisa Page - which the inspector general had addressed in a previous report.

Among the texts Graham pointed to was an August 2016 message in which Strzok told Page he was at a Walmart in southern Virginia and could "smell the Trump support." The text, Graham said, showed those with key roles at the FBI did not want Trump to win. "These are a few bad people that couldn't believe Trump won, didn't want him to win, and when he won, couldn't tolerate the fact that he won," Graham said. "And all these smelly people elected him. This is bad stuff."

The inspector general's report has exposed major disagreements among Trump, Barr, Horowitz and FBI Director Christopher Wray, and lawmakers are likely to press the inspector general further on the areas where there are disputes.

Horowitz concluded the FBI had a valid reason to open an investigation that in its early days scrutinized four Trump campaign advisers: Page, George Papadopoulos, Paul Manafort, and Michael Flynn.

In the fall of 2016, FBI officials sought and received court approval to conduct electronic surveillance on Page, after he left the campaign, suspecting he might be an agent of the Russian government. The inspector general concluded FBI agents "failed to meet the basic obligation" to ensure the applications for surveillance on Page were "scrupulously accurate."

Horowitz found "so many basic and fundamental errors were made by three separate, hand-picked teams on one of the most sensitive FBI investigations that was briefed to the highest levels within the FBI," that there was a failure of "not only the operational team, but also of the managers and supervisors, including senior officials, in the chain of command."

Much of the FBI's surveillance application for Page was based on a now-infamous dossier - a collection of lurid allegations against Trump, prepared by a former British intelligence officer hired by an opposition research firm working for Hillary Clinton's presidential campaign. Graham said he first saw the document after it was passed to him from Sen. John McCain, who got it from someone else at a national security conference in Canada in December 2016. He said his initial reaction was "this could be Russian disinformation," or Russians could "have something on Trump." McCain died in August 2018.

But over time, he would learn, "It's a bunch of crap." And he said the FBI would eventually learn that, too, but never told the court.

Trump hailed the inspector general's report as vindication for his long-running accusations that the FBI's former leaders schemed to carry out a coup to remove him from office - accusations Horowitz's findings implicitly reject.

But Barr, the attorney general, said Horowitz's criticisms did not go far enough, telling The Wall Street Journal on Tuesday that the FBI's handling of one of the most sensitive political investigations it has ever conducted was "a travesty," and that Durham continues to pursue the matter.

Wray, the FBI director, announced the bureau would implement more than 40 changes within the bureau, particularly when it comes to how it handles informants and applications to the Foreign Intelligence Surveillance Court.

But Wray's comments in a television interview angered the president, who tweeted Tuesday morning: "I don't know what report current Director of the FBI Christopher A. Wray was reading, but it sure wasn't the one given to me. With that kind of attitude, he will never be able to fix the FBI, which is badly broken despite having some of the greatest men & women working there!"

An FBI spokeswoman declined to comment.