Earlier this month, the supreme court justice Clarence Thomas put out a statement in which he addressed the storm of criticism that has engulfed him following the blockbuster ProPublica report that revealed his failure to disclose lavish gifts of luxury vacations and private-jet travel from a Texan real estate magnate.

Thomas confirmed that the Dallas billionaire and Republican mega-donor Harlan Crow and his wife Kathy were “among our dearest friends”. Thomas admitted, too, that he and his wife Ginni had “joined them on a number of family trips during the more-than-a-quarter-century we have known them”.

The justice, who is the longest-serving member of the nation’s highest court and arguably its most staunch conservative, insisted he had taken advice that “this sort of personal hospitality from close personal friends” did not have to be reported under federal ethics laws. He emphasized that the friend in question “did not have business before the court”.

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But a close look at Thomas’s judicial activities from the time he became friends with Crow, in the mid-1990s, suggests that the statement might fall short of the full picture. It reveals that a conservative organization affiliated with Crow did have business before the supreme court while Thomas was on the bench.

In addition, Crow has been connected to several groups that over the years have lobbied the supreme court through so-called “amicus briefs” that provide legal arguments supporting a plaintiff or defendant.

In 2003, the anti-tax group the Club for Growth joined other rightwing individuals and organisations, including the Republican senator Mitch McConnell and the National Rifle Association (NRA), in attempting to push back campaign finance restrictions on election spending.

At the time of the legal challenge, from at least 2001 to 2004, Crow was a member of the Club for Growth’s prestigious “founders committee”. Though little is known about the role of the committee, it clearly commanded some influence over the group’s policymaking.

During the course of a 2005 investigation into likely campaign finance violations by the Club for Growth, the Federal Election Commission (FEC) noted that rank-and-file club members could “vote on an annual policy question selected by the founders committee”.

Crow has also been a major donor to the club, contributing $275,000 to its coffers in 2004 and a further $150,000 two years later.

The 2003 legal challenge championed by the Club for Growth targeted the McCain-Feingold Act, which had been passed with cross-aisle backing the previous year. The legislation placed new controls on the amount of “soft money” political party committees and corporations could spend on elections.

On appeal, a consolidated version of the lawsuit, Mitch McConnell v FEC, was taken up by the supreme court. In a majority ruling, the court allowed the most important elements of the McCain-Feingold Act to stand (though they were later nullified by the supreme court’s contentious 2010 Citizens United ruling).

Thomas was livid. He issued a 25-page dissenting opinion that sided heavily with the anti-regulation stance taken by the Club for Growth and its rightwing allies. Thomas began his opinion by breathlessly accusing his fellow justices of upholding “what can only be described as the most significant abridgment of the freedoms of speech and association since the civil war”.

By the time Thomas issued his opinion in December 2003 he had already forged his deep relationship with Crow. According to the billionaire, they first met at a conference in Dallas in 1994 – by which time Thomas had already been nominated by George HW Bush to the most powerful court in the land.

The businessman had already showered Thomas with several lavish gifts before the McCain-Feingold challenge reached his court. Thomas disclosed for instance a 1997 flight from Washington to northern California on Crow’s private jet to attend an all-male retreat at Bohemian Grove at which the justice went on to become a regular guest.

There was also a Bible once owned by Frederick Douglass, then valued at $19,000. In 2001 Crow made a $150,000 donation to create a Clarence Thomas wing within the Savannah, Georgia, library the justice frequented as a child.

The federal law 28 US Code section 455 requires any federal judge – including the nine supreme court justices – to recuse themselves from any proceeding “in which his impartiality might reasonably be questioned”.

ProPublica’s explosive investigation earlier this month exposed undeclared gifts and travel that have continued to be bestowed by the billionaire on Thomas to this day. They included a nine-day vacation with Ginni in Indonesia in the summer of 2019 the cost of which probably exceeded $500,000.

In a later report, ProPublica revealed that in 2014 Thomas sold his mother’s home in Savannah to Crow. That transaction was also left undisclosed.

The ProPublica disclosures have prompted a debate about the need for greater scrutiny of the conduct of supreme court justices. Top Democrats have called for an official inquiry into Thomas’s behavior and for all the justices to be subject to a strict ethics code.

The progressive Democratic congresswoman Alexandria Ocasio-Cortez, speaking on CNN, decried Crow’s largesse as “very serious corruption” and called for Thomas to be impeached.

Gabe Roth, executive director of Fix the Court, a non-partisan group which advocates supreme court reform, said that a crisis of trust in Thomas’s ethical judgments had been bubbling below the surface for some time. “The reason that it is so salient now is that the supreme court has grown exponentially in power since Justice Thomas took that first private plane ride in 1997 – when the court becomes the most powerful government body, then ethics issues become all the more critical.”

The Guardian contacted Thomas at the supreme court but did not receive a response.

This week, the normally media-shy Crow, who has assets valued at $30bn and who has donated at least $13m to Republicans, gave an in-depth interview to the Dallas Morning News. He claimed the furore around his relations with Thomas was a “political hit-job” by the liberal media.

He insisted he and Thomas were just friends who spent their time talking about their kids and animals. “We talk about dogs a lot,” he said.

Asked whether he ever considered their friendship as a ticket to quid pro quo, he replied: “Every single relationship – a baby’s relationship to his mom – has some kind of reciprocity.”

Crow’s office, in a statement to the Guardian, disputed any relevance of Crow’s links with the Club for Growth, his friendship with Thomas, and the justice’s opinion in the McConnell v FEC case. “Harlan Crow was not a party to the litigation, was only a financial supporter of Club for Growth, and had no role whatsoever in any Club for Growth litigation decisions.”

The statement continued: “Any insinuation that Justice Thomas wrote his opinion in this case because Harlan Crow was a supporter is ridiculous as Justice Thomas had already expressed these same views in a previous case, Nixon v Shrink MO PAC.”

The billionaire’s office insisted that Thomas’s skepticism of the constitutionality of campaign finance regulation “was established before he had even met Harlan Crow”.

Crow has never personally come before the supreme court, and denies ever trying to influence Thomas on any legal or political issue. But he has served on the boards of at least three conservative groups that have lobbied the supreme court through amicus briefs. Early in his friendship with Thomas, Crow sat on the national board of the now defunct Center for the Community Interest, which filed at least eight amicus briefs in supreme court cases backing rightwing causes such as sweeping crime off the streets and countering pornography.

He has also been a trustee for more than 25 years of the American Enterprise Institute, a thinktank advancing free enterprise ideas that has filed several supporting briefs to the court. In 2001 AEI gave Thomas a bust of Abraham Lincoln then valued at $15,000.

Crow is an overseer of the Hoover Institution, a conservative thinktank based at Stanford University. In February, Hoover senior fellows led an amicus brief filed to Thomas and his fellow justices challenging the $400bn student loan debt-relief program introduced by Joe Biden.

The supreme court is likely to rule on whether the scheme can go ahead this summer. In oral arguments in February, Thomas was among the rightwing justices who hold the supermajority who indicated they were skeptical of the program, raising the possibility that the court will scupper the hopes of more than 40 million Americans eligible for the debt relief.