Trump told reporters Friday that he didn't speak to Whitaker about the probe.
The event was part of nationwide protests organized in response to recent shakeups in the administration. But now, Mueller will need to get Whitaker's sign-off for those and other major steps - which he could withhold.
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. He said there was no evidence of collusion.
Over the course of his investigation thus far, Mueller and his team of prosecutors indicted 32 individuals and three Russian businesses on charges ranging from computer hacking to conspiracy and financial crimes.
Crowds also turned out in Chicago; Greensboro, N.C.; Chattanooga, Tenn., and many other places.
The move means that Whitaker will oversee Robert Mueller's investigation into potential coordination between Russian Federation and Trump's 2016 campaign.
Whitaker has the authority to fire Mueller, but he could also simply reassign some or all of the 13 career prosecutors who now serve on Mueller's team.
"According to the chain of command, (Rosenstein) should be the attorney general", Gorton said. In many instances, that office does not require a Justice Department official to recuse, but suggests a course of action.
To this point, Trump has heeded advice not to directly interfere, though a new chapter in the relationship with the probe may have begun with the appointment of Matthew Whitaker as new acting attorney general. We targeted the opioid epidemic by prosecuting doctors, pharmacists, and anyone else who contributes to this crisis with new law enforcement tools and determination.
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La apertura de la cuenta fue con un golazo de Teo Gutiérrez , figura del pleito, al recibir al límite, sacarse al portero y definir al arco vacío.
"America is better than this", said Christina Kalenderian, a 33-year-old writer from Medford.
But Tuesday's victory by Democrats in capturing the House has consequences here.
Sessions' resignation, in a one-page letter to President Donald Trump, came one day after Republicans lost control of the House of Representatives and was the first of several expected post-midterms Cabinet and White House departures.
The White House says the press is blowing a statement by a private citizen out of proportion, and President Trump himself insists he's never spoken with Whitaker about the probe. But Trump has long sought greater visibility into the probe.
Whitaker's selection by Trump as acting attorney general drew sharp criticism from Democratic lawmakers, who described it as an attempt by the president to undermine and possibly end Mueller's investigation.
Whitaker has publicly spoken about possible ways to slow or obstruct Mueller's investigation. According to the department's most recent report, $10 million was spent from October 1, 2017, through March 31.
Department of Justice spokesperson Sarah Isgur Flores declined to comment.
"Committees of the United States Congress are conducting investigations parallel to those of the special counsel's office, and preservation of records is critical to ensure that we are able to do our work without interference or delay", the Democrats wrote.