Trump, who denies any collusion, has long complained about the probe, calling it a witch-hunt.
The White House selected Whitaker without seeking the consent of Congress, which may present its own Constitutional issues.
They insisted on anonymity to comment on conversations they were not authorized to describe.
As Mueller has gotten closer to the heart of the Trump family's closely guarded financial secrets, the president has become more enraged.
"We are in the lowest of the low times", Samuelian said.
"The attorney general receives a regular flow of information about the special counsel's actions; he can demand an explanation for any of them; and he has power to intervene when he deems it appropriate to prevent a deviation from established Departmental practices", Mueller wrote.
As attorney general, and earlier as White House counsel, he faced criticism for drafting controversial rules for military war tribunals, seeking to limit the legal rights of detainees at the USA prison in Guantanamo Bay, Cuba.
Gonzales, who was the attorney general in George W. Bush's administration from 2005 to 2007, said he doesn't know Whitaker, his qualifications or motivations, but there's a lot that calls into question his appointment. What? Because he has an impression what the Mueller investigation should look like is different from what Senate and House Democrats believe?
A clear focus concerns Stone, a longtime political dirty trickster. He said it did not take a lawyer or former federal prosecutor to recognize that any inquiry into Trump'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 has his supporters too, including a US senator from Iowa, Joni Ernst.
Bannon and other Stone associates have been questioned, and multiple witnesses have appeared before the grand jury.
Mueller's team is paid for under a permanent fund for investigations by independent counsels that was established in 1998, which means it's not subject to Congress's routine spending debates. Whitaker has publicly outlined strategies to stifle the investigation and can not be allowed to remain in charge of it.
The protests come a day after Trump effectively fired Attorney General Jeff Sessions, a move many see as clearing the way for Trump to go after Mueller.
One organizer for Capital Area Indivisible, Jessica Gorton of Readfield, has helped organize other area events for the group, including the 2017 Women's March.
He bolstered the agenda of Trump's evangelical supporters with new rules letting employers opt out of providing health insurance that covers birth control and paving the way for religious businesses and employees to discriminate based on their closely held beliefs.
But with Sessions being pushed out yesterday, that oversight now reverts to his replacement, his former chief of staff, and now the acting attorney general, Matthew Whitaker.
The switch, announced abruptly in a Trump tweet, provoked consternation across Washington, where politicians from both sides of the aisle have long warned that political interference in Mueller's work can not be tolerated.
Whitaker had previously criticized Mueller's probe.
A separate letter from the Democrats called on Whitaker to recuse himself and keep the Mueller investigation under the supervision of Deputy Attorney General Rod Rosenstein, a career Justice Department official who has given the special counsel full scope to pursue leads. Whitaker has made no public comment since being named, but is reported in U.S. media to have made clear he will not recuse himself. And not everyone is convinced he needs to.
Senate Democratic leader Chuck Schumer called on Whitaker to recuse himself over his previous statements and Democratic Representative Jerrold Nadler said the House Judiciary Committee would investigate Session's firing once he assumes the chairmanship in January.