WASHINGTON (AP) — Supreme Court Justice Clarence Thomas failed to publicly disclose additional travel on GOP megadonor Harlan Crow's private jet, a top Democratic senator who is backing an election-year push to tighten the high court's ethics rules said Monday.

Thomas and his wife, Virginia, took a round-trip flight between Hawaii and New Zealand in November 2010, according to international flight records maintained by Customs and Border Protection, Sen. Ron Wyden, D-Ore., wrote.

The letter to Crow's lawyer is part of an inquiry that Wyden, the chairman of the Senate Finance Committee, opened after several reports that Thomas had for years received undisclosed luxury travel from Crow.

Wyden's letter demands additional information about private jet and superyacht trips. "I am deeply concerned that Mr. Crow may have been showering a public official with extravagant gifts, then writing off those gifts to lower his tax bill," he wrote. The letter was first reported by The New York Times.

A spokesman for Crow said he has always followed tax law and Wyden's inquiries have "no legal basis." "It's concerning that Senator Wyden is abusing his committee's powers as part of a politically motivated campaign against the Supreme Court," said spokesman Michael Zona in a statement.

The Senate Judiciary Committee previously uncovered at least three additional trips in its own investigation, including to Indonesia and California.

Democrats, including President Joe Biden, are separately pushing for changes to the high court, and seeking to capture voter attention in an election year after ethical revelations about some justices, including trips by Thomas and Justice Samuel Alito.

In recent days, Justice Elena Kagan has voiced support for an enforceable ethics code for her and her colleagues. Justice Neil Gorsuch, in an interview with The Associated Press last week, declined to comment on proposals on ethics and term limits for the high court.

Justices fill out financial disclosure forms each year detailing any outside sources of income or gifts. The form Thomas filled out in 2010 does not list any flights aboard Crow's jet, Wyden wrote.

Thomas did not immediately respond to a request for comment sent to the court.

The justice, 76, has maintained that he is not required to disclose the many trips because they amount to personal hospitality from a close friend without business before the court.

The November 2010 trip came a month after Thomas flew to the capital of Trinidad and Tobago, on a private plane owned by his friends, Anthony and Beatrice Welters, the Times has previously reported. Anthony Welters was serving as the American ambassador to the country in the Caribbean, having been appointed by President Barack Obama.

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Associated Press writers Mary Clare Jalonick and Mark Sherman contributed to this report.