Special counsel Robert Mueller's investigation into President Donald Trump has returned to the limelight following Tuesday's midterms, and the battles with the White House are now likely to heat up fast.

Trump fired the first shot as the probe returns to the forefront of the political sphere on Wednesday when he announced that Attorney General Jeff Sessions resigned and Sessions' chief of staff, Matthew Whitaker, was elevated to acting attorney general.

In his resignation letter to Trump, Sessions wrote he resigned "at (Trump's) request," making it clear he was ousted from his role.

Whitaker, in media appearances and op-eds, has been critical of the Mueller probe. Now, he'll be overseeing it, as a Department of Justice spokeswoman essentially confirmed in a Wednesday statement.

Up until now, that oversight role was performed by Deputy Attorney General Rod Rosenstein because Sessions recused himself from all matters related to the 2016 presidential campaign.

Mueller would most likely submit a report upon the completion of the investigation to Whitaker, if he remains in his position. It is not known whether that report will be provided to Congress or made available to the public, and Whitaker could simply decline to provide anyone with the report.

The House Intelligence Committee, soon-to-be in Democratic control, could subpoena the report and anyone involved in preparing it, however.

For much of the past two months prior to Wednesday, Mueller's probe into Russian electoral interference and possible ties between Trump's campaign and Moscow went quiet, as had been expected. Department of Justice guidelines recommend not taking any law enforcement actions within 60 days of an election to avoid influencing voters or even giving the appearance of swaying them.

The battle most observers were paying attention to prior to the relatively quiet period involved whether Trump will sit for an interview with Mueller. That fight is now almost certain to ratchet up once again.

In August, Trump's lawyers were already preparing a memo to oppose a subpoena from Mueller, which the special counsel could serve to compel the president to sit down for such an interview.

"We would move to quash the subpoena," Trump's personal attorney Rudy Giuliani told The Washington Post at the time. "And we're pretty much finished with our memorandum opposing a subpoena."

Such a back-and-forth could set off a monumental legal fight in federal court, possibly going all the way to the Supreme Court.

The discussion of a possible subpoena came as both sides continued an extended back-and-forth over the conditions for an interview. The issue, as Giuliani has made clear in multiple interviews, involves whether Trump will answer any questions on potential obstruction of justice regarding the ongoing Russia probe and if the president will provide Mueller with written answers or will sit for an interview.

Trump's legal team wants the questioning to focus exclusively on possible links between the Trump campaign and Russia and not delve into obstruction. They have also stated their preference for written answers by Trump to Mueller's questions.

Trump's lawyers, however, have long cautioned the president against providing Mueller with any interview whatsoever, saying that the former FBI director may lure him into a perjury trap.

During a recent interview with Fox News host Laura Ingraham, Trump said he would "probably" agree to answer some of Mueller's questions after the election.

Last week, CNN reported that Trump has started to review with his legal team the answers he plans to provide to Mueller's written questions, though no determination has been made about the president sitting with the special counsel for an in-person interview. CNN also noted that Mueller's team has started to write its final report.

Andrew Wright, who was associate counsel to President Barack Obama, told NBC News that it would make sense for a Trump interview to be among the last steps Mueller takes in the investigation before completing a report on his findings.

"I would expect that to come to pass at the end of the process, if (the interview) happens," Wright said, noting that investigators prefer to interview a key figure like Trump after they've completed all of their other investigatory work.

Meanwhile, there were some developments to come out of the Mueller investigation during the two-month quiet period.

In September, Trump's former campaign manager Paul Manafort — a central figure in the investigation — reached a plea deal with Mueller. Manafort is set to be sentenced in February.

And just last week, NBC News and The New York Times reported on how longtime Trump associate Roger Stone and right-wing conspiracy theorist Jerome Corsi have found themselves at the center of the investigation. NBC News reported that Mueller's office obtained communications that suggested Corsi, whom Mueller subpoenaed in September, may have had advance knowledge that emails from Hillary Clinton's campaign chairman John Podesta were stolen and provided to WikiLeaks.

Stone says he only passed along information already in the public domain, now insisting he never provided the campaign with inside information about the Podesta emails ahead of their publication.

Renato Mariotti, a former federal prosecutor, told NBC News he expects Mueller to ramp up his efforts regarding Stone following the election.

"It appears to me that Mueller is closing in on Stone, interviewing witnesses close to him," Mariotti said. "I wouldn't be surprised if Stone is indicted soon. Based on his public statements, Stone wouldn't be surprised either."

Since his investigation began last year, Mueller has filed more than 100 criminal charges against 32 individuals. The special counsel has secured guilty pleas or convictions from every U.S. person charged as a result of the probe.

That includes former national security adviser Michael Flynn, who cooperated with prosecutors and is set to be sentenced next month, and former Trump campaign advisers Rick Gates and George Papadopoulos. Mueller has also charged 13 Russian nationals and 12 Russian intelligence officers with crimes including conspiracy to defraud the U.S. and conspiracy to commit an offense against the U.S.