The preparations for a Democratic takeover have been underway for months.

One source familiar with the discussions said that "it would have been malpractice" not to be ready even as leadership encouraged members to exercise caution. The person described rigorous planning in which key oversight teams were communicating with each other "every single day."

"Winning the majority is a mandate to provide a check and balance in the form of oversight and accountability that's been completely absent during two years of the Trump administration under Republican control of Congress," said. Rep. Gerry Connolly, a Democrat from Virginia. "But how we do it is what will be the test. We can't look like Torquemada in the Spanish Inquisition. It has to be fact-based, methodical, meticulous and well-grounded. And judicious. But I believe we are more than capable of doing that. We've done it before."

Where to look

The House Oversight and Government Reform committee will be at the center of the action. Elijah Cummings, a Democrat from Maryland who is expected to lead the committee, plans to look at "all the things the President has done that go against the mandates of our founding fathers in the Constitution."

"Right now, we have a president who is accountable to no one," Cummings told CNN.

Still, Cummings insisted he would "work very hard" to approach his chairmanship in a deliberative and bipartisan manner. "I don't want people to think we are going to rush in and beat up on Trump," he said.

Even in the minority, Cummings sought to investigate potential violations of the Emoluments clause and whether the administration followed protocols when it came to their employees' security clearances. In the weeks leading up to the election, Cummings has accused Trump of being far more instrumental than first thought in the decision to keep the FBI headquarters in downtown Washington, DC, as opposed to moving it to the suburbs, a move Democrats argued was so that Trump could ensure another developer wouldn't buy it and build something that would compete with Trump's nearby hotel.

But, the House Judiciary Committee will also have a major stake in the oversight game. Over the last few months, the panel's Democrats have sent dozens of letters on everything from the Trump administration's family separations on the Southern border to the rise of white nationalism to why the Justice Department has refused to defend the Affordable Care Act in a Texas court case.

Nadler, whose political fights with Trump stretch back decades over New York real estate politics, told CNN that his committee would probe many of those same issues they previously pressed Republicans to examine, including family separation, gun safety, environmental laws and the Justice Department's failure to defend the Affordable Care Act.

"He's going to learn that he's not above the law," Nadler said of Trump.

Trump's tax returns

Another top priority will be asking for Trump's tax returns. Rep. Richard Neal, the man expected to lead the House Ways and Means Committee, told CNN in October he plans to first ask Trump for them. If that fails, he will use an arcane IRS code to formally request them, a move that is expected to launch a months-long court battle.

"I think we would all be comfortable if this was done on a voluntary basis," Neal said. "If they would resist the overture then I think you could probably see a long and grinding court case."

Cummings said Trump's tax returns would "probably" be pursued as part of his panel's Emoluments investigation as well, but predicted that the committees would coordinate their oversight efforts. "The last thing we want to do is step on each other," he said.

"All of this is complicated because it's like coming upon an 88-car pile-up on the highway. It's hard to know where to begin," said Maryland Rep. Jamie Raskin, a member of the Judiciary and Oversight Committees.

For policy committees, expect even more oversight of federal agencies. The House Energy and Commerce committee will conduct oversight into the ways the Trump administration has weakened protections for people with pre-existing conditions at the Department of Health and Human Services. And, the House Natural Resources Committee's expected Chairman Raul Grijalva has said he wants to bring Interior Secretary Ryan Zinke before his committee after reporting that the Department of Justice is investigating the secretary.

"Secretary Zinke will be called to testify in February on why his conduct in office merited referral to the Justice Department, whether that referral was related to the recent attempted firing of his inspector general, and his many other failures and scandals," Grijalva said in a statement before the midterms.

Russia

The new Democratic majority is also likely to result in a restarting of the congressional investigation into potential collusion between Trump's team and Russia, which House Republicans concluded in March.

But Rep. Adam Schiff, who is likely to become chairman of the House Intelligence Committee, isn't planning to re-launch a full blown investigation into Russia. That's because special counsel Robert Mueller is believed to be close to completing his work, and the Senate Intelligence Committee is also nearing the finish line of its own Russia investigation.

As a result, Democrats plan to wait to see what Mueller and the Senate find -- and what questions they believe are still unanswered, according to a senior House Democratic aide. They expect there could be several key issues that might go unanswered that they can continue to probe, including potential Russian money laundering, Trump's financial ties to Deutsche Bank and the number Donald Trump Jr. called when he dialed a blocked phone the while arranging the June 2016 Trump Tower meeting.

"The question, though, that I don't know whether Mueller has been able to answer, because I don't know whether he's been given the license to look into it, is were the Russians laundering money through the Trump Organization?" Schiff told CNN's Wolf Blitzer last month. "And that will be a very high priority to get an answer to. For the reason that, if they were doing this, it's not only a crime, but it's something provable."

Of course, one unanswered question that could change Democrats' planning is that they don't know what form the end result of the special counsel will take, if his findings will even be provided to Congress and who might be supervising the special counsel investigation if Attorney General Jeff Sessions or Deputy Attorney General Rod Rosenstein leave the Justice Department.

DOJ regulations don't require that Mueller's findings are provided to Congress, and if they are not sent to Capitol Hill, it's likely to be one of the biggest early fights between the new Democratic House and the Trump administration.

Will the White House cooperate?

One outside White House ally predicted an "long fight" over subpoenas from Democratic investigations between lawmakers and the Trump administration, but said the White House might be inclined to cooperate more than some might expect.

"Why give them a needle when you can give them the haystack?" the ally said, suggesting that document dumps might drown Democrats in so much paperwork that it keeps them occupied for months.

Separately, another White House official acknowledged that they will need a coordinated campaign between legal and communications teams to fight back in public. Much of this response is expected come down to what new White House counsel Pat Cipollone advises, the official said.