Senate Judiciary Committee Democrats have sent letters to two wealthy businessmen and a major political activist requesting more information about undisclosed gifts to Supreme Court justices.

The letters, sent Tuesday by Sen. Sheldon Whitehouse, D-R.I., and Sen. Dick Durbin, D-Ill., the committee chair, seek more details about an undisclosed 2008 luxury fishing vacation Justice Samuel Alito took that was reported last month by ProPublica. The letters went to three people: hedge fund billionaire Paul Singer; mortgage company owner Robin Arkley II; and Leonard Leo, a longtime leader at the Federalist Society, the powerful conservative legal group.

All three men played a role in paying for or organizing Alito’s 2008 vacation, but the letters go beyond that trip. The senators requested Leo and the businessmen provide a full accounting of all transportation, lodging and gifts worth more than $415 they’ve ever provided to any Supreme Court justice.

“To date, Chief Justice Roberts has barely acknowledged, much less investigated or sought to fix, the ethics crises swirling around our highest Court,” Durbin and Whitehouse said in a joint statement. “If the Court won’t investigate or act, Congress must.” The senators’ committee has announced it plans to vote on July 20 on a bill that would tighten Supreme Court ethics rules.

A spokesperson for Singer said he had received the letter and was in the process of reviewing it. Leo declined to comment but previously said that Alito could never be influenced by a free trip. Arkley and the Supreme Court press office did not immediately respond to requests for comment.

ProPublica reported last month that Singer flew Alito on a private jet to a luxury Alaska fishing vacation in July 2008. Alito did not pay for the trip, including his stay at the fishing lodge, which was owned by Arkley, a significant conservative political donor. Leo helped organize the trip and asked Singer if Alito could fly on the billionaire’s jet. The justice did not disclose the gift of the private jet trip in his annual financial disclosure, which ethics law experts said appeared to be a violation of federal ethics law.

In the years following the trip, Singer’s hedge fund had cases come before the court at least 10 times. Alito did not recuse himself. He ruled with the court’s majority in favor of Singer’s hedge fund in a 2014 case that pitted the fund against the nation of Argentina.

Alito wrote in a Wall Street Journal op-ed published before the ProPublica story that he had not known Singer was affiliated with the hedge fund, and he maintained that disclosure rules didn’t require him to report the private jet flight. A spokesperson for Singer said last month that the billionaire had “never discussed his business interests” with the justice and that Singer had not organized the trip.

The letters sent Tuesday represent a new phase in the Senate investigation of Supreme Court ethics.

This spring, ProPublica reported that Justice Clarence Thomas received decades of unreported gifts from Dallas real estate billionaire Harlan Crow. Crow took Thomas on private jet flights and yacht cruises around the world, paid private school tuition for the justice’s grandnephew and paid Thomas money in an undisclosed real estate deal. The Senate Judiciary Committee launched an investigation and wrote a series of letters to Crow, demanding a full accounting of his gifts to Thomas and any other justices over the years.

Thus far, Crow has resisted the senators’ probe. The billionaire’s lawyers have argued that Congress does not have the authority to investigate the gifts and that the inquiry violates the separation of powers. Thomas has defended himself by saying he took family trips with friends. Crow has said he never discussed pending legal matters with Thomas or sought to influence him.

Leo also joined Crow and Thomas during at least one undisclosed trip to the billionaire’s private resort in the Adirondacks. A painting Crow commissioned depicts Leo at the resort alongside the justice and the billionaire. In the new letter, the senators asked the longtime Federalist Society executive to provide details about any travel he’s ever taken with any Supreme Court justice.

The expanded investigation comes as the Senate Judiciary Committee prepares to vote on Supreme Court ethics reform. Following the Alito report, Durbin and Whitehouse announced that the panel would vote on a reform bill this month.

“To hold these nine Justices to the same standard as every other federal judge is not a radical or partisan notion,” Durbin and Whitehouse said in a joint statement, adding, “The belief that they should not be held accountable or even disclose lavish gifts from wealthy benefactors is an affront to the nation they were chosen to serve.”

The bill, titled the Supreme Court Ethics, Recusal, and Transparency Act, would significantly tighten ethics rules but in many cases leave the details up to the court itself.

The bill requires the court itself to create and publish a code of conduct within 180 days but doesn’t lay out in detail what rules it should contain. Lower court federal judges are already subject to a code of conduct, but it does not apply to the Supreme Court.

In other areas, the bill is more specific: It would tighten recusal rules, including in cases when justices accept gifts from litigants at the court or affiliates of litigants. If the proposed law had been in place when Alito sat on Singer’s case against Argentina, it appears it would have required the justice to recuse himself.

The bill would also require the court to create an ethics complaint process. Members of the public could submit complaints and investigations would be carried out by a randomly selected panel of five appellate judges. The panel could recommend that the Supreme Court take disciplinary action. It could also publish reports of its findings.

Under current law, justices are not required to — and rarely do — explain themselves when they do or don’t recuse themselves from a case. It’s a long-standing parlor game among Supreme Court watchers to guess what conflict or potential conflict led a justice to recuse himself or herself. The bill would end that. It would require published written explanations of recusal decisions.

The bill would also tighten some rules around the disclosure of gifts and of the funding behind friend-of-the-court briefs that are filed by outside groups in many high-profile cases.

The bill is already facing steep opposition, with influential Republicans in both the House and Senate coming out against legislative reforms. Minutes after Durbin announced the committee vote, the Twitter account for the Republicans on the House Judiciary Committee responded: “And that’s as far as it will go. God Bless Justice Alito!”

The response among Republican lawmakers has not been uniform, however. Sen. Lisa Murkowski, R-Alaska, introduced a bill this year that would require the court to adopt a code of conduct and create a process for investigating potential violations of it. Other Republican senators have encouraged Chief Justice John Roberts to take action to tighten the court’s ethical standards himself.

Sen. Cynthia Lummis, R-Wyo., told The Hill following the recent Alito revelations that she believes it’s in the Supreme Court’s “best interests to address this issue to the satisfaction of the public and use the standards that should apply to anyone in the executive or legislative branch with regard to ethics.”