President Donald Trump has tapped former US attorney Matthew Whitaker to be acting attorney general following Jeff Sessions' ouster on Wednesday.

Whitaker has been described as the White House's "eyes and ears" in the Justice Department, which is investigating whether the Trump campaign colluded with Moscow during the 2016 US election.

Whitaker frequently meets with Trump at the Oval Office and has publicly decried Special Counsel Robert Mueller for going too far in the Russia investigation.

When President Donald Trump announced on Wednesday that Attorney General Jeff Sessions was out as the head of the Justice Department, all eyes turned to the man who will temporarily replace him: Matthew G. Whitaker, Sessions’ former chief of staff at the DOJ.

Whitaker, a former US attorney from the Southern District of Iowa, is viewed by many as a staunch Trump loyalist.

According to The New York Times, White House chief of staff John Kelly once described Whitaker as the West Wing’s “eyes and ears” in the DOJ, which is currently investigating whether the Trump campaign colluded with Russia to tilt the 2016 election in his favor.

Whitaker was reportedly previously being considered to replace Deputy Attorney General Rod Rosenstein, a frequent target of Trump’s wrath, which stems from his belief that the DOJ isn’t doing enough to shield him from Special Counsel Robert Mueller, who is spearheading the FBI’s Russia investigation. But Trump and Rosenstein are said to have soothed their differences in recent weeks after the two had a series of one-on-one meetings and phone calls.

Whitaker, meanwhile, reportedly visits the Oval Office frequently. In September, amid speculation that Trump would oust Rosenstein, the president called Whitaker himself to assure him that he had faith in him, according to The Times.

At the heart of Trump’s frustration with Sessions was the former attorney general’s decision last year to recuse himself from the ongoing Russia investigation after it surfaced that he had misled the Senate about his Russia contacts during his confirmation hearings.

Trump — who once reportedly mused about why “my guys” at the “Trump Justice Department” weren’t protecting him — has repeatedly said that if he had known Sessions would step back from the investigation, he would have tapped someone else to be attorney general.

With Whitaker as acting attorney general until Trump nominates a replacement, the president appears to have focused on picking someone he believes will exercise more influence over Mueller and the Russia probe.

To be sure, Whitaker wrote in a CNN op-ed shortly before he was hired as Sessions’ chief of staff that “Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing.”

Whitaker added that his concerns stemmed from reports that the special counsel was probing the Trump Organization’s financial records.

“It does not take a lawyer or even a former federal prosecutor like myself to conclude that investigating Donald Trump's finances or his family's finances falls completely outside of the realm of his 2016 campaign and allegations that the campaign coordinated with the Russian government or anyone else,” Whitaker wrote. “That goes beyond the scope of the appointment of the special counsel.”