Nadler and other Democrats on the House Judiciary Committee also sent letters to Whitaker and the top Republican on the committee, retiring Rep.
Sessions, a Republican and former Alabama senator, has been the focus of public attacks by Trump, who criticized him for recusing himself from the Russian Federation investigation because of his role on the Trump campaign team before becoming attorney general.
Whitaker said during his time as a commentator for the CNN cable television network that the Mueller probe should be limited, and that if Sessions were fired then a new attorney general could cut Mueller's funding to the point the investigation would come to a halt.
Mr Whitaker expressed concerns over the investigation.
The Justice Department has declined to answer whether ethics officials have been asked to review if Whitaker should recuse himself from the Mueller probe.
Trump said he will nominate a permanent successor for Sessions at a later date.
The special counsel has also been investigating whether Mr Comey's firing amounted to obstruction of justice. "The American people understand that no person is above the law and have demanded accountability from their government". If Mr Mueller files a report detailing his conclusions, the new acting attorney general could keep the document from ever becoming public.
Hajime Tabata, director de Final Fantasy XV, se va de Square Enix
Hoy los jugadores de Final Fantasy XV esperaban tener buenas noticias sobre los contenidos descargables previstos para el juego. Lo anterior los obligó a analizar nuevamente los proyectos a futuro para finalmente optar por la cancelación de los DLCs .
The senator said he found the timing of Sessions' departure "very suspect" and suggested it would cause a "constitutional crisis" if Trump made the move as a "prelude" to limiting the Russian Federation investigation.
Can Whitaker be forced to recuse himself?
In her tweet, Pelosi, who is likely to become House speaker under the new Democratic-controlled Congress, wrote that "It is impossible to read Attorney General Sessions' firing as anything other than another blatant attempt by Donald Trump to undermine & end Special Counsel Mueller's investigation".
Sessions took himself out of decision-making on the probe in March 2017 even after Trump dispatched the White House lawyer to persuade him not to do so, according to a person familiar with the matter. "I'm not recused, you're not recused", Conway told reporters.
He also said on CNN past year that he could see a scenario in which Sessions' replacement doesn't fire Mueller but "just reduces his budget to so low that his investigation grinds to nearly a halt". Donald Trump himself seems to be aware of the consequences, saying at a press conference on Wednesday: "I don't want to stop it because politically I don't want to stop it".
And there are thousands and thousands of career officials, agents, and prosecutors who swore an oath to the Constitution, and I trust and have faith that they're going to - they're going to do their allegiance to that oath.