In an op-ed in August a year ago he publicly urged Deputy Attorney General Rod Rosenstein - who oversees the probe - to "limit the scope of his investigation to the four corners of the order appointing him special counsel".
But it's not typical for chiefs of staff to replace the head of an agency when that head resigns, as former CIA staffer and NBC contributor Ned Price pointed out.
"The forced firing of Attorney General Sessions appears to be part of an ongoing pattern of behavior by the president seeking to undermine (the) investigation into Russian interference", said the letter to Goodlatte, written by Representative Jerrold Nadler, the committee's top Democrat, and 16 other Democrats who sit on the panel. "That's why Cabinet Secretaries have Deputies".
It wasn't clear immediately why Trump chose the hours after midterm results to fire Sessions.
Justice Department rules on special counsels set boundaries on how Mueller could be removed.
The decision infuriated Trump, who repeatedly lamented that he would have never selected Sessions if he had known the attorney general would recuse.
Sessions, who likely suspected his ouster was imminent, was spotted by reporters giving some of his grandchildren a tour of the White House over the weekend.
Matthew previously ran as a republican for U.S. Senate and state treasurer. "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", he wrote.
Iran sanctions had 'enormous' economic consequences: John Bolton
Tehran branded the sanctions economic warfare and vowed to defy the measures as the U.S. threatened further action. The reinstated sanctions follow the Trump administration's decision in May to pull out of the global nuclear deal.
Mr Sessions removed himself from the probe after Democrats accused him of failing to disclose contacts he had had with the Russian ambassador as a senior adviser to Mr Trump's campaign.
"Trump has done a naked attempt to mess with the investigation", she said.
"If he were to continue to investigate the financial relationships without a broadened scope in his appointment, then this would raise serious concerns that the special counsel's investigation was a mere witch hunt", Whitaker wrote.
When asked if he would fire Mueller, as some critics have feared, Trump said, "I could have ended it any time I wanted, [but] I didn't".
But Senate Judiciary Chairman Chuck Grassley praised Whitaker.
Sessions, a 71-year-old former Republican senator from Alabama, was the first senator to endorse Trump's insurgent presidential campaign.
One reason Sessions endured months of taunting from Trump is that he believed he was protecting the integrity of the Justice Department and was trying to prevent the president from bringing in someone who would politicize it, according to a USA official who has worked with Sessions and frequently talked with him.
By Thursday morning, more than 1,000 people said on a Facebook page for the event that they planned to attend, and more than 3,200 said they were interested.