Jeff Sessions resigned from his role as US attorney general in a letter sent to the White House on Wednesday.
"A permanent replacement will be nominated at a later date", he said.
"The dismissal of the nation's top law enforcement official is a huge step, one that should not be based on political motives - and certainly should not be done to protect the president or his cronies from the law", Romero said.
As a member of Trump's 2016 campaign, Sessions was forced to recuse himself from the Justice Department's investigation of ties between that campaign and the Russian government.
Mr Rosenstein in turn appointed Mr Mueller, a former Director of the Federal Bureau of Investigation.
Appearing on CNN in July 2017 - before he was in his current position as Sessions's chief of staff - Whitaker mused about a scenario in which Trump might fire Sessions and replace him with a temporary attorney general.
Senate Minority Leader Chuck Schumer said after the news broke that Trump risked creating a "constitutional crisis" if he impeded Mueller's investigation, Reuters reported.
Democrats have moved gingerly around Sessions - fearful that if he were driven from office, his replacement could curtail the Russian Federation investigation led by Special Counsel Robert Mueller.
"Any investigation into President Trump's finances or the finances of his family would require Mueller to return to Rod Rosenstein for additional authority under Mueller's appointment as special counsel", he wrote.
An element of Mr Mueller's investigation includes whether Trump or anyone in the campaign tried to obstruct justice.
Odd inherited Sessions's Senate seat after his appointment as U.S. Attorney General. Sessions' recusal prevented him from stopping the probe, which Trump has attacked as an unfair "witch hunt".
Sessions said was "honored to serve" and said his Justice Department "restored and upheld the rule of law - a glorious tradition that each of us has a responsibility to safeguard".