Now that Democrats control the House of Representatives, they can use their new subpoena power as a check on President Donald Trump.

Observers say the Democratic victory could not have come at a better time, given that Trump ousted Attorney General Jeff Sessions one day after the midterms.

Not only could Sessions be one of the witnesses Democrats call to testify in the House Russia investigation once they relaunch it, but his willingness to cooperate with investigators may have also gone up following his dismissal.

"For two years, the Trump Administration has been able to operate without any check-and-balance on their affairs," a former spokesperson for Republicans on the House Oversight and Government Reform Committee told Insider. "Now, every meeting, email, appointment ... is subject to congressional oversight. This will have a seismic effect on the Trump administration."

One of the major criticisms of the Republican-controlled House of Representatives over the last two years has been its apparent protection of President Donald Trump from scrutiny over his campaign's contacts with Russia-linked individuals during the 2016 election.

On Tuesday, Democrats flipped the House following the midterm elections and effectively cut off that buffer.

Many Trump critics say the timing couldn't have been better, because the very next day, Trump ousted embattled Attorney General Jeff Sessions and replaced him with a hardline loyalist who has publicly railed against the special counsel Robert Mueller and speculated about how to gut the Russia investigation.

In the wake of Sessions' dismissal, perhaps the most crucial weapon at Democrats' disposal is the ability to subpoena documents and witnesses relevant to any pending or future investigations into the executive branch.

Sessions has long been one of the central figures in the congressional and FBI Russia probes. Legal experts told Insider that not only could Sessions become one of the witnesses Democrats call to testify, but his willingness to cooperate with investigators may have also increased following his removal.

Kurt Bardella, a former spokesperson for Republicans on the powerful House Oversight and Government Reform Committee, didn't mince words when discussing the importance of subpoena power.

"The Democrats gaining subpoena authority is the most consequential effect of the elections," Bardella told Insider. "When Republicans started the inquiry into Benghazi, they had no idea it would lead to the discovery that former Secretary of State Hillary Clinton was using a private email server. Imagine how different the 2016 campaign would have been if 'Hillary's emails' had not been an issue."

The House Oversight Committee has already compiled a list of 64 subpoenas and inquiries looking into Trump administration activities that Democrats say Republicans didn't allow them to investigate before.

And a Democratic source close to the House Intelligence Committee told Insider that issuing subpoenas to individuals and entities they were previously barred from contacting will be central to Democrats' game plan once they start reopening investigative threads in the panel's Russia probe that they say Republicans ignored.

'We have to keep all these seats'

House Intelligence Committee Chairman Devin Nunes, right, and ranking member Rep. Adam Schiff. Mark Wilson/Getty Images

The House Intelligence Committee has been the epicenter of partisan bickering over the Trump-Russia controversy for nearly two years. Though Chairman Devin Nunes and Ranking Member Adam Schiff first pledged to work together to investigate the Trump campaign's Russia contacts, the probe soon devolved into two separate investigations.

While Democrats pushed to secure documents and question witnesses about Trump's links to Russia, Republicans launched a parallel investigation into purported surveillance abuses and misconduct at the Department of Justice and FBI. The Republican inquiry was largely led by Nunes — even though he recused himself from the investigation last year — and wielded significant influence through the use of subpoenas.

In particular, Republicans subpoenaed thousands of pages of classified Russia documents from the DOJ and later released many of them to the public with the White House's backing.

Meanwhile, earlier this year, Schiff laid out a detailed list of the individuals and entities he said Nunes refused to allow Democrats to question in the House Russia probe. These included Deutsche Bank for records of financial ties between Trump and Russian interests, executives at Twitter over communications between Trump associates and WikiLeaks, first daughter Ivanka Trump, the Russian lawyer Natalia Veselnitskaya, and others.

Schiff also told The Washington Post that Republicans refused to press the attendants of a June 2016 Trump Tower meeting, which was held to get dirt on Clinton, for a complete and unredacted record of their communications.

Now that the balance of power has shifted in the lower chamber, two sources close to the intelligence and judiciary committees told Insider Democrats will make it a top priority to focus their resources on digging into the Trump-Russia controversy, with an emphasis on financial ties between Trump and Russian interests. And Schiff released a statement this week saying Trump's links to Deutsche Bank will be at the top of the agenda.

Nunes, for his part, appeared to acknowledge the gravity of a potential Democratic takeover of the House earlier this year.

"If [Attorney General Jeff] Sessions won't un-recuse and Mueller won't clear the president, we're the only ones, which is really the danger ... we have to keep all these seats," Nunes can be heard saying on a recording from a private fundraiser that aired on MSNBC's Rachel Maddow Show.

'This will have a seismic effect on the Trump administration'

Ousted Attorney General Jeff Sessions. Alex Wong/Getty Images

But Republicans critical of Mueller notched a significant victory when Trump ousted Sessions on Wednesday and replaced him with his chief of staff, Matthew Whitaker.

Whitaker, a former US attorney from the Southern District of Iowa, is viewed by many as a stalwart Trump supporter who was once reportedly described by White House chief of staff John Kelly as the West Wing's "eyes and ears" in the DOJ.

A department spokesperson told Insider in an email that Whitaker will assume oversight of the FBI's Russia investigation and Mueller from Rosenstein.

DOJ veterans immediately sounded the alarm, citing Whitaker's past statements and coziness with Trump as reasons the acting attorney general should recuse himself from overseeing the Russia probe, like Sessions did last year.

Democratic lawmakers also leapt into the fray, with several signaling that they would use their new investigative authority as a check on Trump's power.

"Why is the president making this change and who has authority over Special Counsel Mueller's investigation?" Democratic Rep. Jerry Nadler, the ranking member on the House Judiciary Committee, said in a tweet. "We will be holding people accountable."

Schiff said in a statement that it is "abundantly clear" Sessions was forced out for recusing himself from the Russia investigation. Schiff also criticized Whitaker for being "publicly critical" of Mueller and said the acting attorney general should recuse himself from overseeing the Russia investigation based on his comments.

"For two years, the Trump administration has been able to operate without any check-and-balance on their affairs," Bardella said. "Now, every meeting, email, appointment, etc. is subject to congressional oversight. This will have a seismic effect on the Trump administration."

Trump, for his part, appears to be gearing up for battle as well. The president said during a freewheeling press conference Wednesday that he would retaliate if Democrats used their subpoena power to obtain his tax returns or reopen the House Russia investigation, adding that he would adopt a "warlike posture" to get Senate Republicans to investigate the Democrats in turn.