A new report from the liberal ProPublica reporting consortium alleges that Associate Justice Thomas has for decades, without disclosing it, embarked on lavish holidays at little or no cost to him with a prominent Republican billionaire, potentially violating a law that requires federal officials to report such excursions as gifts.

The report said Justice Thomas has traveled aboard private jets and yachts owned by a Dallas businessman who is a founding member of the conservative Club for Growth political action committee that backs GOP candidates across the country, Harlan Crow, and regularly holidays at estates and vacation homes owned by Mr. Crow. The trips reportedly include multi-day cruises around the Indonesian archipelago and New Zealand, as well as stays at private resorts in the Adirondacks and Mr. Crow’s East Texas ranch.

ProPublica said none of the trips were listed on the financial disclosure forms that Justice Thomas files. Ethics experts and current and former federal judges cited by the report called Justice Thomas’s actions problematic and a deviation from long-standing norms for federal judges. A former government ethics lawyer quoted by the outlet, Virginia Canter, said the associate justice “seems to have completely disregarded his higher ethical obligations.”

“When a justice’s lifestyle is being subsidized by the rich and famous, it absolutely corrodes public trust,” Ms. Canter, who is now at the watchdog group Citizens for Responsibility and Ethics in Washington, which frequently aligns itself with liberal advocacy groups, said.

Justice Thomas did not respond to questions from the outlet, but a statement provided by Mr. Crow said he and his wife have been “very dear friends” with Justice Thomas and his wife, Virginia, better known as Ginni, for decades.

The hospitality they have extended to the Thomases over the years “is no different from the hospitality we have extended to our many other dear friends,” the statement said. “We have never asked about a pending or lower court case, and Justice Thomas has never discussed one, and we have never sought to influence Justice Thomas on any legal or political issue.”

The friendship between the Thomases and Crows has been reported previously, most notably in a New York Times article in 2011 about Mr. Crow’s donations to a Georgia museum championed by the Supreme Court justice. Other reports have documented Mr. Crow’s donations to conservative activist groups that employed Ms. Thomas, but the ProPublica report is the first to note the two families’ frequent holidays together.

The report comes just days after Senate Democrats ramped up their efforts to force the Supreme Court to adopt a code of ethics for its justices. The court has been considering adopting a code of ethics for years, but has to date not been able to reach a consensus on the issue. The justices say they voluntarily comply with ethical guidelines that apply to other federal judges.

The pressure campaign to force the court to adopt more transparent rules regarding recusals and gifts and other potential influences has picked up steam since President Trump managed to appoint three conservative justices during his four-year term and reverse the previous court’s more liberal inclinations. Ms. Thomas’s public comments decrying the outcome of the 2020 presidential election only stirred the pot.

Senator Van Hollen, a Maryland Democrat, told the Washington Post earlier this week that he and his colleagues on the Senate Appropriations Committee are considering using this year’s spending bill to force the Supreme Court to adopt and adhere to an enforceable ethics code similar to the one followed by federal judges.

“The Supreme Court should have a code of ethics to govern the conduct of its members, and its refusal to adopt such standards has contributed to eroding public confidence in the highest court in the land,” Mr. Van Hollen said. “It is unacceptable that the Supreme Court has exempted itself from the accountability that applies to all other members of our federal courts, and I believe Congress should act to remedy this problem.”