The Washington firm that commissioned the dirty dossier on President Trump is stonewalling congressional investigators.

In a piece for the New York Post, conservative author Paul Sperry writes that the firm, Fusion GPS, which was started by three former Wall Street Journal reporters, is not cooperating with Congress in order to cover up an allegiance to the Democratic party.

Sen. Chuck Grassley, R-Iowa, the chairman of the Senate Judiciary Committee, is spearheading an investigation into the dossier, and wants to know the identities of the people who paid Fusion to dig up dirt on Trump.

Congressional sources told Sperry that Fusion GPS's founders are more political activists than journalists, who have a pro-Hillary Clinton, anti-Donald Trump agenda.

'These weren't mercenaries or hired guns,' a congressional source familiar with the dossier probe told the writer. 'These guys had a vested personal and ideological interest in smearing Trump and boosting Hillary's chances of winning the White House.'

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Just off of Dupont Circle is the office building where Fusion GPS has its headquarters. Republican Sen. Chuck Grassley is trying to get the firm to divulge who paid them to dig up dirt on Donald Trump during last year's presidential campaign

Sen. Chuck Grassley, chairman of the Senate Judiciary Committee, has written letters to Fusion's Glenn Simpson, asking for information about the 'dirty dossier'

Former Wall Street Journal reporter Glenn Simpson is one of the co-founders of Fusion GPS, which tapped British ex-spy Christopher Steele who assembled the 'dirty dossier' on President Donald Trump

Simpson's co-founders included other Wall Street Journal alumni including Peter Fritsch (left) and Thomas Catan (right)

Fusion tapped Christopher Steele, an ex-M.I.6 Moscow field agent, to probe his sources in Russia to compile the 35-page dossier, filled with salacious allegations about the now-president, which haven't been proven.

Sperry writes that Fusion GPS was on the payroll of an unnamed Democratic ally of Clinton's when Steele was tapped to look into Trump, but before that, as a Vanity Fair cover story on the matter points out, it was Republicans who were behind Fusion's initial interest in Trump.

British ex-spy Christopher Steele was tapped by Fusion to look into Trump-Russia links, as he had formerly been based in Moscow

In September 2015, Glenn Simpson, one of the Wall Street Journal reporters who started Fusion, was hired to compile opposition research on the longtime businessman, several months after he jumped into the presidential campaign.

Simpson wouldn't reveal to Vanity Fair his client's identity, but a friend of the ex-reporter who talked to him around that time told the magazine the funding came from a 'Never Trump' Republican, and not from one of Trump's primary opponents, like Sen. Ted Cruz or Sen. Marco Rubio.

By mid-June of 2016, Trump had locked the nomination up, and the 'Never Trump' Republican was done handing money to the firm.

At the same time, the Washington Post had a story out, 'Inside Trump's Financial Ties to Russia and his Unusual Flattery of Vladimir Putin.'

'Simpson, as fellow journalists remember, smelled fresh red meat. And anyways, after all he had discovered, he'd grown deeply concerned about the prospect of a Trump presidency,' Vanity Fair wrote.

So at that point, Simpson found a Democratic donor who would continue to pay for the research – and he gave Steele a buzz, knowing the ex-spy had strong Russia ties.

The Post's Sperry writes that the firm's founders had more than just their commitment to a Democratic client motivating them to dig up dirt on Trump.

Simpson, for instance, had done opposition research in the past for a former Clinton White House operative.

Fusion co-founder Peter R. Fritsch contributed at least $1,000 to the Hillary Victory Fund and the Hillary For America campaign, Sperry found. His wife, Beatriz Garcia, also donated to the Democratic nominee.

Sperry pointed out that Fritsch married into a family with Mexican business interest, as his wife was a former employee of truck and bus manufacturer Grupo Dina, which benefits from NAFTA, a policy that Trump hates.

Fusion GPS's third partner, Thomas Catan, is British, but also lived in Mexico editing a business magazine. While Fritsch had been the Wall Street Journal's bureau chief in Mexico City.

DailyMail.com's own reporting on the firm found that they had done work to advance President Obama's presidential campaign in 2012, by digging up dirt on a principle donor who was assisting the Democrat's rival, Mitt Romney.

Fusion also did work for Planned Parenthood.

Now, the Senate Judiciary Committee is investigating whether the FBI, which received a copy of the dossier in August, improperly relied on it in the agency's investigation into Trump and Russia, now being handled by Special Counsel Robert Mueller.

The FBI was reportedly going to pay Steele $50,000 to corroborate what his Russian sources had told him.

Senate Judiciary Committee Chairman Chuck Grassley noted that relationship 'raises substantial questions about the independence' of the FBI in its Trump investigation, as the dossier was being funded by Democrats.

'The dossier was the product of blatant political and financial motives,' Grassley's office said in a press release making public what the senator had sent to Fusion GPS.

In a letter to Simpson, Grassley requested that Fusion reveal the identities of both the research firm's clients, the Republicans who initially wanted opposition research done on Trump and then the Democrats who later funded Steele's work.

Grassley also had a number of questions about who the dossier was distributed to and how it got to the FBI.

Fusion gave the senator nothing.

'You refused to provide any information whatsoever, claiming that the Committee's request "calls for information and documents protected by the First Amendment right, attorney-client privilege, attorney work product, and contractual rights (e.g. confidentiality agreements) of Fusion and/or its clients,"' Grassley wrote in a follow-up.

'However, in both your response letter and on a subsequent phone call with Committee staff, your attorney refused to provide a clear explanation of the basis for the claimed privileges and rights, and has failed to provide any privilege log describing the withheld documents,' Grassley continued.

The Iowa Republican explained that even if Fusion had once been protected by confidentiality agreements, the firm waved those rights once it began sharing the dossier to journalists, that privilege was waved.

'It hardly seems plausible that Fusion's client funded opposition research with the intention of keeping the discovered information confidential, especially based on Fusion's efforts to share the dossier with journalists and members of Congress,' Grassley said.

Fusion didn't respond to a request for comment from DailyMail.com, asking if the firm had complied with Grassley since this last exchange, nor to a question about the firm's political motivations, which were outlined by the New York Post.