U.S. Attorney John Durham’s investigation into the intelligence and law enforcement communities' handling of the Russia probe is quietly but steadily expanding under the shadow of the high-profile House impeachment inquiry against President Trump—and could represent something of a wild card in the president’s attempts to fight back.

Attorney General Bill Barr appointed Durham, the U.S. attorney from Connecticut, to ensure intelligence collection activities by the U.S. government related to the Trump 2016 presidential campaign were “lawful and appropriate.”

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His probe reportedly will soon focus on the roles of key Obama administration intelligence officials like John Brennan and James Clapper. And it converges with other simultaneous investigations, including Justice Department Inspector General Michael Horowitz’s probe of alleged Foreign Intelligence Surveillance Act (FISA) abuses, which, coupled together, could give the president ammunition to attack his critics, even if the material does not directly relate to the Ukraine controversy fueling the impeachment push.

“If the rumors are true that IG Horowitz’s report and findings in Durham’s review will blast the conduct of the FBI’s Russia investigation, it will give Trump a lot of ammo to support his argument that he was unjustly targeted then and is being unjustly targeted now,” a House GOP source told Fox News on Tuesday. “It will justify Trump’s warnings about the Deep State acting to hobble his presidency.”

Trump claimed Tuesday that the impeachment push amounted to a "lynching" -- which touched off a round of fiery condemnation from Democratic critics.

“The president should not compare a constitutionally mandated impeachment inquiry to such a dangerous and dark chapter in American history,” House Democratic Caucus Chairman Hakeem Jeffries, D-N.Y., told reporters.

But as Trump and Democrats loudly clash over the probe, Durham has pressed forward quietly with an investigation that could ding the reputations of some of Trump's biggest critics.

Durham was appointed to review the events leading up to the 2016 presidential election and through Trump’s January 20, 2017 inauguration. But Fox News has learned that he's since expanded his investigation to cover a post-election timeline spanning the spring of 2017—when Robert Mueller was appointed as special counsel.

Durham is “gathering information from numerous sources, including a number of foreign countries,” according to Justice Department spokeswoman Kerri Kupec, who also acknowledged that Trump has helped to facilitate communications for Barr and Durham with foreign powers.

“At Attorney General Barr’s request, the president has contacted other countries to ask them to introduce the attorney general and Mr. Durham to appropriate officials,” Kupec said last month.

Barr and Durham have already traveled to Italy to speak with law enforcement officials, and have also had conversations with officials in the U.K. and Australia about the probe, according to multiple sources familiar with the meetings.

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Durham also has reportedly expressed interest in interviewing several current and former intelligence officials. Former CIA Director John Brennan told NBC News that Durham plans to interview him and former Director of National Intelligence James Clapper.

A spokesman for Clapper did not immediately respond to Fox News’ request for comment.

Meanwhile, Horowitz is expected to release his long-awaited review of alleged FISA abuses by the Department of Justice and the FBI during the Russia investigation—there has been speculation that his report could drop any day.

Horowitz, for more than a year and a half, has been investigating alleged misconduct related to the FISA warrants delivered by the Foreign Intelligence Surveillance Court. The Justice Department and FBI obtained warrants in 2016 to surveil Trump adviser Carter Page. It is unclear, at this point, if Page was the only Trump campaign official that the DOJ obtained a FISA warrant against.

Horowitz’s highly anticipated findings could spark new congressional investigations and deliver critical information to Durham’s probe.

Horowitz has been probing how the salacious anti-Trump dossier compiled by former British spy Christopher Steele was used to secure the original FISA warrant for Page in October 2016, as well as three renewals. Horowitz also has looked into why the FBI may have regarded Steele – funded by the Hillary Clinton campaign through law firm Perkins Coie – a credible source, and why the bureau used news reports to bolster Steele’s credibility before the FISA court.

“As soon as Horowitz is done with his review of the FISA warrant application, the counterintelligence investigation, the Trump campaign, we’ll have a hearing in public with Horowitz and we’ll call a bunch of witnesses,” Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., said on Fox Business Network’s “Sunday Morning Futures.”

Graham has vowed to probe alleged FISA abuses at the start of the Russia investigation, saying earlier this year that his Senate committee would look for answers on how much money the Democrats paid research firm Fusion GPS to commission the dossier compiled by Steele, or if the contents of the dossier have been verified.

It is unclear if Graham, amid the House impeachment inquiry, has begun this investigation. But Graham has said that he could call on Justice Department official Bruce Ohr and former FBI Director James Comey to appear before his panel.

The president and his allies are already touting the progress being made by Durham, and are hoping Horowitz’s report will provide new fodder to counter impeachment talk.

“Democrats are afraid that the reports will validate what the president has been saying for years—his enemies in Congress are so desperate to undo the results of the 2016 election that they will manufacture conspiracies and sell them to the American people,” a senior Republican aide told Fox News on Tuesday.

Trump has the authority to declassify and release as much of the report as he wants, and has been hyping its forthcoming release.

“I predict you will see things that you don’t even believe, the level of corruption—whether it’s [James] Comey; whether it’s [Peter] Strzok and his lover, [Lisa] Page; whether it’s so many other people—[Andrew] McCabe; whether it’s President Obama himself,” Trump told reporters last week.

“Let’s see whether or not it’s President Obama. Let’s see whether or not they put that in,” he added.

The president has sought to shift focus on the current impeachment inquiry in the House to potential misconduct that could be found in these ongoing investigations.

House Speaker Nancy Pelosi, D-Calif., announced the formal process last month, following revelations surrounding the president’s summer phone call with Ukrainian President Volodymyr Zelensky in which he pressed for politically charged investigations.

As detailed in a whistleblower complaint and transcript of the call, Trump pushed the Ukrainian president to launch an investigation into former Vice President Joe Biden and his son, Hunter Biden, over their dealings in Ukraine—specifically, why the elder Biden pressured the former Ukrainian president to fire a top prosecutor who was investigating a natural gas firm where Hunter sat on the board.

The president’s request also came after millions in U.S. military aid to Ukraine had been frozen, something critics have cited as evidence of a quid pro quo arrangement. The White House and the president’s allies have denied a quid pro quo -- though Acting White House Chief of Staff Mick Mulvaney seemed to say otherwise, before walking it back -- and the Bidens have maintained that they did “nothing wrong.”

Fox News' Jake Gibson, Bret Baier, and Gregg Re contributed to this report.