As Democrats mull how far to push the impeachment envelope against President Trump after Robert Mueller found no evidence of collusion with Russia in the 2016 election but punted on obstruction of justice charges, another investigation could further blunt their attempts to oust the president from office or damage his re-election chances.

Amid calls from Trump and his supporters to “investigate the investigators,” Justice Department Inspector General Michael Horowitz has been hard at work over the last year looking into the sources and methods the FBI used to begin surveillance of a one-time Trump campaign adviser based at least in part on discredited information gathered by a former British spy.

That packet of intelligence, known as the “Steele dossier,” contains salacious and unsubstantiated details about Trump’s alleged romps with Russian prostitutes, along with business and political quid pro quos with Russian officials.

Attorney General Robert Barr said the inspector general is wrapping up his probe and could release a final report as early as next month.

Those interviewed by Horowitz and his team over the past year, according to Politico, say he seems intensely focused on undermining the dossier and credibility of Christopher Steele, the former British MI6 agent who produced the document. Steele had served as a confidential source for the FBI since 2010 until a falling out over his leaks to the media about the Trump-Russia probe.

While prominent Democrats have accused Mueller of failing to do his duty and Barr of prioritizing the interests of Trump over the American people, they’ll have a more difficult time assailing Horowitz, a Harvard-educated lawyer appointed by President Obama to the DoJ’s top watchdog post in 2012.

Horowitz, who was a partner in New York City’s oldest law firm, Cadwalader, Wickersham & Taft LLP, before becoming inspector general, served as a board member of the Ethics Resource Center and the Society of Corporate Compliance Ethics.

He began his career at the Justice Department in the 1990s, serving as an assistant U.S. attorney for the Southern District of New York, including stints as the chief of the public corruption unit. Before leaving in 2002, he worked as the deputy assistant attorney general of the criminal division and as its chief of staff.

Roughly a year ago, Horowitz also proved he’s willing to disappoint Trump and his supporters. He thoroughly investigated the FBI’s handling of the Hillary Clinton email investigation and charges that the probe was rigged to let Clinton off the hook.

Horowitz amassed a mountain of embarrassing emails and electronic messages between former FBI counterintelligence agent Peter Strzok and his co-worker and lover, Lisa Page, about their hatred for Trump and an “insurance plan” to derail his presidency. However, Horowitz concluded that he could not link the “appearance” of personal bias against Trump to “evidence that any political bias or improper considerations actually” impacted the way the FBI pursued the Clinton email probe.

He also harshly criticized then-FBI Director James Comey for his July 2016 announcement that he would not recommend any charges against Clinton, and his subsequent October 2016 decision to tell Congress that the FBI had discovered new emails and had re-opened the case.

Still, Horowitz concluded that Comey hadn’t acted out of political bias, but did “deviate” from established procedures and engaged “in his own subjective, ad hoc decision making” in what the IG described as an extremely unusual case with high political stakes.

The stakes couldn’t be higher when it comes to Horowitz’ current probe. Steele was hired by the opposition research firm Fusion GPS in 2016 to look into Trump’s Russia ties, and that work was funded by the Hillary Clinton campaign and the Democratic National Committee through a law firm.

Republican members of Congress and other Trump allies allege the only true collusion took place between the Clinton camp and the FBI, with Steele’s help. They accuse the DoJ and the FBI of abusing the FISA process and misleading the Foreign Intelligence Surveillance Court by relying on the dossier to obtain approvals for the surveillance without disclosing that the information was unverified or paid for by Democrats and the Clinton campaign itself.

Democrats counter that the FBI wouldn’t be doing its job if it hadn’t investigated Trump associates’ ties to Russia. For instance, the unpaid campaign adviser at the center of the FISA controversy, Carter Page, first attracted FBI attention back in 2013 when he interacted with undercover intelligence agents in New York City. Carter’s trip to Russia in the summer of 2016 sparked more scrutiny and justified the warrant the FBI submitted to in October 2016, they argue.

But Trump and his supporters have blasted the FBI for continuing to use the dossier to attain FISA court warrants even after Steele was terminated for unauthorized and potentially criminal leaks to the media.

Last January, then-Senate Judiciary Committee Chairman Chuck Grassley and Sen. Lindsey Graham, who now helms that panel, referred Steele to the Justice Department for criminal prosecution for lying about his contacts with several media organizations before the 2016 election.

Rep. Jim Jordan, who serves as the ranking Republican member on the House Oversight Committee, on Saturday pointed to the dossier as the rationale used to launch an investigation “on a false premise.”

“You can’t have the FBI using one party’s opposition research document to launch an investigation and spy on the other party’s campaign,” he said.

Rep. Matt Gaetz, a conservative Republican from Florida, over the weekend said Horowitz has evidence that FBI officials received tickets to concerts and athletic events from members of the press as incentives to leak to them.

“One of the … nuggets that the inspector general is working on is the corruption that existed between the media and members of the FBI,” Gaetz said, without citing his sources for the information.

The American public, especially those on the right, are already highly skeptical of the mainstream media, whose credibility has continued to sink during its coverage of the Trump administration amid the president’s frequent charges of “fake news” and the media’s torrent of stories alleging Trump’s collusion with Russia.

A Morning Consult/Hollywood Reporter survey released earlier this month found that the share of adults who said some of the biggest media outlets – including ABC, CBS, CNN, Fox News, MSNBC, the New York Times, NPR and the Wall Street Journal – were credible dropped an average of 5 percentage points over the past three years, from 56% to 44%.

The media skepticism was predictably most pronounced among Republicans, whose responses show a 12-point drop in their trust in news outlets over the course of the last three years.

It also doesn’t help that the dossier first surfaced in the liberal media when BuzzFeed posted it online – complete with the lurid details of a sex tape featuring prostitutes that the Russian government was said to be holding over Trump. Mueller’s investigation found no evidence that such a tape existed. It also didn’t corroborate another dossier claim published in a McClatchy report that then-Trump attorney Michael Cohen met with Russian officials in Prague.

The Mueller report’s conclusions poked huge holes in the Democrats’ Trump-Russia narrative and sparked new questions about the way the FBI went about investigating it, as well as the media’s role in fanning its flames. Horowitz’ report will try to address both issues.

At the beginning of the Horowitz probe, House Judiciary Chairman Jerrold Nadler – who is weighing whether to begin impeachment proceedings against the president -- said it’s a “shame” that the inspector general has to “devote resources to investigate a conspiracy theory as fact-free, openly political, and thoroughly debunked as the president’s so-called ‘FISA abuse.’”

As the probe is winding down, Steele himself appears less sanguine about Horowitz’ findings and conclusions. He has reportedly declined to be interviewed and plans to rebut the IG’s characterizations in a rare public statement.

The New York Times on Friday also reported that Steele never tried to portray the dossier as anything other than raw intelligence — jumping off points for the FBI to begin investigating.

How that assertion squares with Horowitz’s findings will be closely watched by those on both sides of the aisle. But for Democrats eager to herald the Mueller report’s details on possible obstruction, the IG’s work could be tough to portray as just another government investigation biased in Trump’s favor.