Getty Images/ AFP/ Bertrand Guay There's been a long-brewing battle between a former banker and a firm that produced the explosive Trump-Russia dossier.

The banker, Bill Browder, testifies before Congress on Thursday.

Republicans in Congress have targeted the firm, Fusion GPS, as the Russia-related investigations heat up.

A fierce but muted battle erupted last year between banker-turned human rights activist Bill Browder and the opposition research firm Fusion GPS, which produced the explosive, unverified dossier that detailed President Donald Trump's alleged ties to and escapades in Russia.

The feud escalated dramatically earlier this month, when news broke of a meeting between the president's son and a Russian lawyer named Natalia Veselnitskaya, who at the time was lobbying to repeal the law known as the Magnitsky Act.

Veselnitskaya's relationship with Fusion's cofounder, Glenn Simpson, has been scrutinized by reporters trying to understand who she is, who she works for, and who works for her. The reporters have had help from Browder, who is known both for spearheading the Magnitsky Act through Congress in 2012 and his willingness to speak to the press.

Asked about Veselnitskaya in an email earlier this month, Browder responded with a PowerPoint presentation that he said highlighted the role she played in trying to get the Magnitsky Act repealed last year. She had help, the slide deck asserts, from some of the "best and brightest minds" in Washington, including Simpson.

Browder's reputation has become inextricably linked to the global human-rights campaign he launched in 2009 after his tax lawyer — or accountant, depending on whom you ask — died in a Russian prison. The lawyer, Sergei Magnitsky, was thrown in jail and beaten to death after he discovered a $230 million tax fraud scheme that implicated high-level Kremlin officials, Browder says.

Others, primarily those sympathetic to Moscow, dispute Browder's story. People close to Simpson contend he is not necessarily one of them. But the two have a somewhat bitter history that has become increasingly politicized amid the intensifying probes into Russia's meddling in the presidential election and the Trump campaign's possible role in it.

Browder is set to testify before the Senate Judiciary Committee on Thursday.

A 'smear campaign' or opposition research?

In 2013, Veselnitskaya hired the law firm BakerHostetler to represent Prevezon, a Russian holding company based in Cyprus accused by the US government of laundering stolen cash into New York City real estate. Browder told prosecutors that the laundered money was stolen from Russia as part of the tax-fraud scheme that Magnitsky uncovered.

To rebut that claim, BakerHostetler hired Fusion to dig up dirt on Browder, he says. The wealthy investor, who renounced his US citizenship in 1998, has since characterized Fusion's work for BakerHostetler as a "smear campaign" against him and Magnitsky carried out "in advance of congressional hearings on the Global Magnitsky Act."

Fusion has denied that its opposition research against Browder on behalf of BakerHostetler was an effort to undermine the Magnitsky Act, and has insisted that Veselnitskaya's lobbying activities against Magnitsky were separate from the Prevezon proceedings. The basis for that claim is that Browder was never able to prove that Prevezon engaged in the tax fraud scheme Magnitsky allegedly uncovered.

But the anti-Magnitsky lobbying activities of Veselnitskaya, who was Prevezon's Russian lawyer, complicate the argument that one had nothing to do with the other. Fusion, Browder says, also pitched anti-Magnitsky stories to American journalists as part of its work for BakerHostetler.

Browder has argued that Fusion would have broken the law if it lobbied on behalf of the Russian government without registering as a foreign agent. In December, he lodged a formal complaint with the Department of Justice alleging that Fusion's work for BakerHostetler on behalf of Prevezon violated disclosure requirements.

Fusion has said that it was hired by an American law firm, which the firm argues would not put it in violation of the Foreign Agents Registration Act as the law currently stands.

"The statute applies to agents, so those who are working at the direction or control of a foreign principle," Adam Hickey, the deputy assistant attorney general for National Asset Protection at the DOJ, told the Senate Judiciary Committee in testimony Wednesday.

The challenge in pursuing charges against people who fail to register, Hickey said, is that often "independent groups may have interests that overlap or coincide with those of foreign governments."

U.S. Sen. Charles Grassley (R-IA). Scott Olson/Getty Images

Enter: Chuck Grassley

In March, Browder's complaint caught the eye of Republican Sen. Chuck Grassley, the chair of the Senate Judiciary Committee.

He wrote a letter to then-Acting Deputy Attorney General Dana Boente demanding to know whether the Justice Department was investigating Fusion for alleged violations of FARA. In the letter, Grassley cited reports that Fusion had been hired by BakerHostetler to investigate Browder, alleging that the work made Fusion "an unregistered agent of Russian interests."

Grassley's interest in the firm wasn't limited to its connection to the Prevezon case, however.

He also wanted the Justice Department to investigate Fusion's role in overseeing "the creation of the unsubstantiated dossier of allegations of a conspiracy between the Trump campaign and the Russians" — the Trump-Russia dossier compiled by former British spy Christopher Steele.

Steele, who served for decades on the MI6 Moscow desk, was hired by Fusion in October 2015 to investigate whether the Trump campaign had ties to Russia. Fusion itself had been hired by unnamed anti-Trump Republicans who dropped their funding for the project when Trump won the GOP nomination. Democrats believed to have links to the Clinton campaign later started to foot the bill.

Grassley cited Browder's complaint against Fusion, and the firm's involvement with the Steele dossier, again two months later during a Senate Judiciary Committee hearing with former FBI Director James Comey.

"According to a complaint filed with the Justice Department, the company that oversaw the dossier's creation was also working with a former Russian intelligence operative on a pro-Russian lobbying project at the same time," Grassley said, alleging that Fusion "failed to register as a foreign agent" for that work.

Glenn Simpson, the co-founder of opposition research firm Fusion GPS. Screenshot/YouTube

Grassley was likely referring to the Russian-American lobbyist Rinat Akhmetshin, a Soviet military intelligence officer who Veselnitskaya and her client, Denis Katsyv, hired last year to lobby against the Global Magnitsky Act. Akhmetshin attended the meeting last June at Trump Tower with Veselnitskaya, Donald Trump Jr., Paul Manafort, and Jared Kushner.

Grassley also questioned during the hearing in May whether Fusion facilitated a form of Russian meddling by contracting out the opposition work to a researcher whose sources were primarily high-level Kremlin officials. The Republican National Committee echoed that talking point later in an attempt to justify Trump Jr.'s meeting with Veselnitskaya as normal opposition research.

"We now know that Democrats actually used a foreign agent to disseminate phony information to the American mainstream media that allegedly came from inside the Kremlin," Michael Ahrens, the director of rapid response for the RNC, wrote in an email sent out to the committee's mailing list earlier this month.

Experts have said, however, that Steele, unlike Veselnitskaya, was not representing a foreign government whose interests he was trying to insert into the election process.

The two projects appeared to have opposite effects, too: Whereas Fusion's work for BakerHostetler was aligned with Kremlin interests, its work for anti-Trump Republicans (and later, for Democrats) reflected badly on the Russian government and bolstered FBI and congressional investigations into Moscow's election interference.

U.S. President-elect Donald Trump speaks with his son Donald Trump Jr. during a news conference in the lobby of Trump Tower in Manhattan, New York City, U.S., January 11, 2017. REUTERS/Lucas Jackson

'Oversight of the Foreign Agents Registration Act'

Browder, Trump Jr., Paul Manafort, and Glenn Simpson were all called to testify before the Senate Judiciary Committee this week about the Foreign Agent Registration Act. Browder and Simpson were likely going to be asked about Fusion, while Manafort and Trump Jr. would face questions about their meeting last June with Veselnitskaya.

Browder is set to testify on Thursday. But Manafort and Simpson have since worked out deals with the committee to either testify behind closed doors or submit written answers to the committee's questions. Trump Jr.'s situation is unclear.

In his written testimony submitted to the committee earlier this week, Browder said that Fusion was hired by BakerHostetler, which was itself hired by Veselnitskaya, "to conduct a smear campaign against me and Sergei Magnitsky in advance of congressional hearings on the Global Magnitsky Act."

Fusion "contacted a number of major newspapers and other publications to spread false information that Sergei Magnitsky was not murdered, was not a whistle-blower, and was instead a criminal," Browder wrote. He added that BakerHostetler was instructed to "to lobby members of Congress to support an amendment taking Sergei Magnitsky’s name off the Global Magnitsky Act."

BakerHostetler did not return repeated requests for comment. In a statement to Business Insider, Fusion said that the president's "political allies" — presumably Grassley, who supported Trump's candidacy — "are targeting Fusion GPS because the firm was reported to be the first to raise the alarm over Trump campaign's links to Russia.”

FILE PHOTO - A combination photo of Donald Trump Jr., Jared Kushner and Paul Manafort Thomson Reuters A spokesperson for Grassley, Taylor Foy, disputed that claim.

"Sen. Grassley has been raising concerns about inadequate enforcement of FARA for years," Foy said in an email on Wednesday.

"Fusion GPS allegedly failed to register under the law for work it was doing to advance Russian interests by repealing sanctions against human rights violators, and it was doing this work at roughly the same time it was producing the unverified Trump dossier, which relies heavily on Russian sources," Foy said. "So of course it would be relevant both to the committee’s review of FARA enforcement and the dossier."

There is another player now alleging that Fusion, in a case completely unrelated to Russia or the Magnitsky Act, acted in violation of FARA.

Thor Halvorssen, the founder of an organization called the Human Rights Foundation, wrote in testimony submitted to the committee on Tuesday that Fusion engaged "in a multi-billion-dollar corruption case benefiting the Venezuelan regime" in 2012.

"In 2012 I began researching a Venezuelan corruption scandal that also involved U.S.banks, companies, and even U.S. courts," Halvorssen wrote. "This story should have received extensive exposure on the front pages of America’s national newspapers. Fusion GPS, however, was hired to spike these stories. Even though it was clearly acting as a public relations counsel on behalf of a foreign principal, Fusion GPS never registered under FARA and was able to engage in nefarious activities without public scrutiny."

Halvorssen alleged in his testimony that he, like Browder, was the victim of a "smear" campaign by Simpson and Fusion. He told Business Insider that Fusion hired journalists to write blog posts with "completely made-up claims," including that he received treatment for a heroin addiction that was paid for by Peter Thiel.

"No proof of these allegations is possible, because they're simply not true," Halvorssen said.

Both Browder and Halvorssen flatly denied in separate interviews that their stories were coordinated in any way.

"When my case became public, I learned about Thor's and I encouraged him to go public with it," Browder said.

Halvorssen's organization lists Browder as a donor, but both maintained that was for a ticket Browder purchased to the HRF's Oslo Freedom Forum in 2016.

Asked whether he harbored a personal vendetta against Simpson for his work with BakerHostetler and, by extension, with the anti-Magnitsky lobbyist Natalia Veselnitskaya, Browder was unequivocal.

"He is a bit player and not important enough for me to have a vendetta against," Browder said. "I have a vendetta against Vladimir Putin, who murdered Magnitsky and covered up the crimes of his officials."