The revelation that Supreme Court Justice Clarence Thomas has accepted free vacations worth hundreds of thousands of dollars — gifted him by a right-wing billionaire and Republican mega-donor — has prompted leading Democrats to demand that the nation's highest court adopt "a proper code of ethics."

In a letter sent Friday and addressed to Chief Justice John Roberts, the top-ranking Democrats on the House and Senate judiciary committees demanded an investigation into Thomas, writing that they were alarmed by the allegations "of unethical, and potentially unlawful conduct at the Supreme Court."

The investigative news outlet ProPublica on Thursday reported that Thomas has for decades now accepted free travel from Harlan Crow, a Texas-based real estate developer who in 2014 purchased a $55 million home in Dallas. One of the vacations cost more than a half million dollars, the outlet reported.

Crow has also given generously — some $500,000 — to a right-wing activist group led by Thomas's wife, Ginni, who in 2020 lobbied state legislators in a bid to overturn the presidential election.

On Friday morning, Thomas himself issued a rare statement in which he described the gifts as "personal hospitality from close friends" and said he was advised that, as such, he did not need to disclose them, CNN reported.

The lawmakers — Sens. Sheldon Whitehouse and Richard Blumenthal, and Reps. Henry Johnson and Jerrold Nadler — are not buying that argument. In their letter, they argue that the trips should have been made public, stating that the disclosure exceptions for federal employees are meant for typical hospitality among "ordinary, personal friendships," not to "allow government officials to hide from the public extravagant gifts by wealthy political interests."

They also call for a "swift, thorough, independent, and transparent investigation into these allegations."

They close by urging Chief Justice Roberts to ensure that the highest court's ethics code is treated are more than just a suggestion.

"It is well past time for the Supreme Court to align with the rest of government in a proper code of ethics enforced by independent investigation and reporting," the letter states, threatening congressional action if it should not do so.

In February, the lawmakers introduced legislation that would require the Supreme Court to create a process for fielding public ethics complaints, with a panel of lower-court judges empowered to investigate them.

Liberal activist groups, who have long been raising concerns about Thomas and his wife's activism, are urging Congress to step in.

"The highest court in the land should have the highest ethical standards," Kyle Herrig, president of the advocacy group Accountable.US, said in a statement. "When it doesn't, Congress should exert its oversight authority."

A spokesperson for the Supreme Court did not immediately respond to a request for comment.