Washington (CNN) The law firm for the Hillary Clinton campaign and the Democratic National Committee has acknowledged its clients' role in paying for opposition research on Donald Trump that helped fund the now-infamous dossier of allegations about the now-President and Russia.

A source familiar with the matter told CNN on Tuesday that the law firm Perkins Coie, as part of its representation of the Clinton campaign and the DNC, retained the intelligence firm Fusion GPS and entered "into an engagement for research services that began in April 2016 and concluded before the election in early November."

Fusion GPS is fighting a legal battle to prevent Fusion's bank from providing the House intelligence committee records that the panel subpoenaed. The letter was filed with the court on Tuesday.

Trump told reporters on Wednesday that the campaign and DNC's involvement with the dossier was a "very sad commentary on politics in this country."

"Hillary Clinton always denied it. The Democrats always denied it, and now, only because it is going to come out in a court case, they said yes they did it. They admitted it and they are embarrassed by it. I think it is a disgrace," he said.

Gehringer wrote that in March 2016, Fusion approached Perkins Coie to see if its clients would be interested in paying Fusion "to continue research regarding then-presidential candidate Donald Trump, research that Fusion GPS had conducted for one or more other clients during the Republican primary contest." The DNC and Clinton campaign engaged Fusion in April to "perform a variety of research services during the 2016 election cycle."

As CNN has previously reported , the Fusion efforts researching Trump were first funded by Republican foes of Trump, and Democrats began paying the research firm later on, after Trump became the presumptive nominee. The identity of the Republican client or clients has not yet been publicly revealed. Without noting the party affiliation, Gehringer calls for other Fusion presidential campaign clients to step forward as well.

"We believe it would be appropriate for all parties who hired Fusion GPS in connection with the 2016 presidential campaign to release Fusion GPS from this obligation," he wrote.

The Washington Post was first to report news of the Clinton campaign and DNC's involvement with the dossier, and a source familiar with the matter confirmed the Post's reporting to CNN on Tuesday.

The newspaper, citing people familiar with the matter, said a lawyer representing the Clinton campaign and the DNC hired Fusion GPS, which in turn hired former British intelligence officer Christopher Steele, whose research makes up the document. The Post said Perkins Coie lawyer Marc Elias retained the research firm in April 2016 as the GOP primary was winding down and Trump was increasingly likely to clinch his party's nomination, and that Fusion GPS hired Steele after the Democratic funding began and the general election picture grew clearer.

Brian Fallon, the Clinton campaign's national press secretary and now a CNN political contributor, tweeted Tuesday that he wasn't aware of the connection between the campaign and the dossier authored by Steele, but "if I had, I would have volunteered to go to Europe and try to help him."

"I have no idea what Fusion or Steele were paid but if even a shred of that dossier ends up helping (Justice Department special counsel Robert) Mueller, it will prove money well spent," he tweeted

DNC communications director Xochitl Hinojosa said in a statement Tuesday: "Tom Perez and the new leadership of the DNC were not involved in any decision-making regarding Fusion GPS, nor were they aware that Perkins Coie was working with the organization. But let's be clear, there is a serious federal investigation into the Trump campaign's ties to Russia, and the American public deserves to know what happened."

And a source familiar with the matter said Elias didn't possess the entire dossier before the election.

"It's important to note that the question raised at the time was if Marc had a 'dossier' before the election and put together pitches to media outlets on information in it. Neither of those statements were true," the source said. "What was eventually published as the 'dossier' by Buzzfeed wasn't in Marc's possession before the election, and like most people, he read it when it was published. While he was certainly familiar with some, but not all, of the information in it, from the research that was being done, he didn't have and hadn't seen the full document, nor was he involved in pitching it to reporters."

But White House press secretary Sarah Sanders said Tuesday that the news of the Clinton campaign and DNC funding the dossier research was "the real Russia scandal."

"The real Russia scandal? Clinton campaign paid for the fake Russia dossier, then lied about it & covered it up," Sanders tweeted.

Subpoenas issued for financial records

Sources told CNN that Mueller's investigators met with Steele over the summer as part of their investigation into Russian interference in the 2016 election and allegations of coordination between Russia and Trump's associates.

The confirmation that the Democratic Party and the Clinton campaign contributed funding that led to the dossier comes as Trump continues to train his fire on the explosive document, most recently tweeting, "Workers of firm involved with the discredited and Fake Dossier take the 5th. Who paid for it, Russia, the FBI or the Dems (or all)?"

Workers of firm involved with the discredited and Fake Dossier take the 5th. Who paid for it, Russia, the FBI or the Dems (or all)? — Donald J. Trump (@realDonaldTrump) October 19, 2017

House Intelligence Chairman Devin Nunes signed subpoenas for Fusion GPS's financial records, and the research firm asked a federal court to block the request, citing First Amendment concerns.

Fusion GPS also claimed that Nunes should have notified it about the subpoena of its bank records.

Nunes stepped aside from the House intelligence committee's investigation into Russia's interference in the election in April following an ethics investigation into his handling of classified information. The California Republican delegated authority on the Russia matter to Republican Rep. Mike Conaway of Texas.

The federal court noted the settlement talks Tuesday afternoon.

Fusion GPS and the bank must let the court know by 3 p.m. Thursday if the parties reach a settlement. The judge in the case also extended a deadline for the bank to reply to the case from Wednesday until Friday morning.

Earlier Tuesday, Fusion GPS lawyer William Taylor issued a statement that Congress' request was overly broad and that it was "designed to punish President Trump's political foes while chilling future investigative research into his actions."

The dossier helped fuel the ongoing investigations into Trump and Russia, including the special counsel probe led by Former FBI Director Mueller. CNN reported in February that investigators had been able to corroborate some information in the dossier, although not the most salacious allegations.

The US intelligence community asserted in January that Russian President Vladimir Putin personally ordered an effort to disrupt the US presidential election.

This story has been updated with details from Gehringer's letter.