Conservative donors poured tens of millions of dollars of anonymous “dark money” into groups supporting Republican lawmakers in a supreme court case that could upend American election law.

The donors backed several groups that have filed supreme court amicus briefs in support of North Carolina legislators in Moore v Harper, according to a recent analysis. They are pushing for a ruling that would take ultimate decisions about voting rights and congressional gerrymandering away from state courts and hand those powers to state legislatures, of which Republicans now control the majority.

Eight conservative groups that submitted amicus briefs in the supreme court case have received close to $90m from dark money donors since 2016, according to Accountable.US, a liberal leaning watchdog group that tracks government corruption.

Several of these conservative bastions are also champions of restrictive voting laws.

Conservatives want the supreme court to adopt the independent state legislature theory, a once fringe idea now promoted by a coterie of conservative groups that filed amicus briefs, including the Honest Elections Project, the Claremont Institute, and the Public Interest Legal Foundation. The groups boast strong ties to rightwing lawyers Leonard Leo, John Eastman and Cleta Mitchell respectively. Eastman and Mitchell were allies in Donald Trump’s baseless crusade to overturn the 2020 election.

Sparked by a North Carolina gerrymandering fight, Moore v Harper has attracted strong opposition from many liberal and some conservative legal experts, who call it a partisan attack on voting rights by prominent conservative groups. Opponents of the case say they’re using a discredited legal theory to boost GOP political fortunes in coming elections.

The leading dark money financier of the conservative groups that filed amicus briefs was DonorsTrust, which contributed a whopping $70.5m, Accountable data shows.

Other top dark money donors to groups that filed amicus briefs include the Lynde and Harry Bradley Foundation and America First Works, which, respectively, gave $6.1m and $4.8m to outfits that supported the independent state legislature theory. The long time conservative Bradley Foundation boasts Mitchell on its board, while the non-profit America First Works has been allied with Trump since its founding in 2016 under another name.

The dark money routed to some of these groups took circuitous routes. For instance, America First Works gave $4.8m to DonorsTrust that was earmarked for the Honest Elections Project, according to Accountable.

The Honest Elections Project, which has been a leading advocate for tougher voting laws in recent years, was founded by Leo, a legendary fundraiser, lawyer and co-chairman of the powerful Federalist Society. Leo was instrumental in advising Trump on his three conservative supreme court nominees.

DonorsTrust, known as the ATM of the right, has been very generous with other projects Leo has helped spearhead. In 2021, for example, Leo’s 85 Fund – a dark money conduit for conservative legal campaigns and other priorities – received its largest single grant of $17.1m from DonorsTrust, which doled out close to $190m that year.

Critics of the right’s drive to push the independent state legislature theory note the strong influence of well-financed conservative groups along with several like-minded justices.

“The ISLT [independent state legislature theory] has been fueled by several conservative justices’ dissents, and other statements, coupled with amicus briefs and public arguments supporting the theory from think tanks, litigation shops, and partisan political organizations,” Thomas Wolf, the deputy director of the democracy program at the Brennan Center for Justice, told the Guardian.

Two key Democrats in Congress, Senator Sheldon Whitehouse and Representative Hank Johnson, submitted an amicus brief arguing forcefully against the independent state legislature theory, highlighting the role of conservative groups funded by dark money who have supported voter suppression efforts.

“Many of the petitioners’ amici actually attempted to undermine the 2020 election by relying on this theory,” Whitehouse and Johnson wrote. “Other amici share connections with groups and individuals who played a role in those attempts. Still others are presently engaged in voter-suppression and election-subversion efforts.

“Rarely has such a noxious assemblage of amici appeared before this court, and their secrecy about their funders and connections does this court a grave disservice,” they added.

The high stakes for democracy behind Moore v Harper and other recent supreme court cases involving dark money funded groups trouble Whitehouse, he said.

In tandem with Johnson, Whitehouse has introduced legislation that would require amicus filers to disclose funders who donated $100,000, or more than 3% of their gross revenues.

In an interview, Whitehouse said his proposed bill coincides with other efforts he has made to have the supreme court change its reporting rules for amicus filers backed by dark money.

“I’ve been pushing the supreme court to update their reporting requirements,” he said about the dark money behind several high-stakes cases, but to date the court has “shown no interest”.

The independent state legislature theory played a key role in Trump’s failed crusade to get states to invalidate the 2020 election results and was the handiwork of Eastman, who filed the amicus brief for the Claremont Institute, a conservative California based thinktank, that made a similar argument.

Eastman’s involvement with Trump’s baseless drive to overturn the 2020 election results, which included promoting an alternative elector scheme to block Congress certifying Joe Biden’s as president, could lead the January 6 panel investigating the Capitol insurrection to file a criminal referral to the justice department for him, as well as Trump and others, according to a recent CNN report.

On a related legal front, Eastman’s refusal to turn over 101 documents to the House panel led federal judge David Carter to rule this year that there was substantial evidence Eastman had conspired with Trump to block Congress from certifying the 2020 election results. The “illegality of the plan was obvious”, Carter wrote.

Just how much the amicus briefs from Claremont and other conservative outfits backed by dark money will influence the supreme court’s ruling on the independent state legislature theory is hard to discern.

Oral arguments in Moore v Harper were heard by the supreme court on 7 December. The court’s three liberal-leaning justices expressed their strong opposition to North Carolina lawmakers’ position, and some conservative justices including Amy Coney Barrett and Brett Kavanaugh also indicated their skepticism about some maximalist versions of the theory.

The genesis of the Moore v Harper case was a ruling by the North Carolina state supreme court in early 2022 that invalidated districts drawn by the Republican-controlled legislature on the grounds they were an “egregious and intentional partisan gerrymander”, unfairly favoring the GOP.

North Carolina legislator Timothy Moore appealed the state supreme court ruling, and a voter named Rebecca Harper was a named plaintiff in a challenge to the state’s gerrymandered maps.

Significantly, North Carolina is one of six states where state courts have ruled in recent years that partisan redistricting plans for Congress violated state constitutions.

Moore v Harper has also sparked significant legal blowback from some prominent lawyers with conservative pedigrees including J Michael Luttig, a former appeals court judge who is a co-counsel for litigants opposing the independent state legislature theory.

“This case swarms with amicus briefs supporting petitioners that elide a salient fact: the doctrine they encourage this Court to adopt – the ‘independent state legislature’ theory – is one of the fringe legal theories deployed in a failed legal plot to overturn the results of the 2020 presidential election,” Whitehouse and Johnson wrote in their brief.