Conservative legal activist Leonard Leo is refusing to provide Democratic lawmakers with information regarding his interactions with Justice Samuel Alito, who attended a luxury fishing trip with him in 2008 and took a ride on a private jet owned by a conservative hedge fund manager.

In a scathing letter Tuesday to key Democrats on the Senate Judiciary Committee, Leo's lawyer said the congressional inquiry "exceeds the limits placed by the Constitution on the Committee's investigative authority" and is tainted by partisan politics. ProPublica first reported the trip, which Alito did not include on his financial disclosure forms.

"By selectively targeting Mr. Leo for investigation on a politically charged basis, while ignoring other potential sources of information on the asserted topic of interest who are similarly situated to Mr. Leo but have different political views that are more consistent with those of the Committee majority, your inquiry appears to be political retaliation against a private citizen in violation of the First Amendment," the lawyer, David B. Rivkin Jr., wrote.

Rivkin was responding to a July 11 letter, penned by Committee Chairman Dick Durbin of Illinois and Sen. Sheldon Whitehouse, a Rhode Island Democrat, asking Leo to provide more information about the trip.

Leo, co-chairman of the Federalist Society, is a longtime player in conservative judicial circles.

A spokesperson for Durbin declined to comment. CNN also has reached out to Whitehouse for comment.

The committee is investigating ethics standards at the Supreme Court as a part of its oversight efforts after an array of stories have been published raising questions of transparency at the Supreme Court. In a statement accompanying the letter earlier this month, Durbin said that "answers to these questions will help the Committee's work to create reliable ethics guardrails at the Court, under Congress's clearly established oversight and legislative authority."

Rivkin said in his letter that the investigation lacks a "valid legislative purpose" because even if it were to produce legislation, it would be "unconstitutional" if enacted, violating separation of powers principles.

He also noted that in the past, Whitehouse has called Leo "the little spider that you find at the center of the dark money web."

Rivkin said that Whitehouse's remarks were akin to a "campaign of innuendo and character assassination" that have now moved "beyond angry speeches and disparaging soundbites."

Leo's attorney also accused the investigation of being "one-sided," noting that while the committee is "professing interest in potential ethics violations and influence-peddling at the Supreme Court," it has focused on individuals "who have relationships with Justices appointed by Republican Presidents."

Rivkin listed allegations against liberal justices that have appeared in the press that he said have been "ignored" by the committee. Among those charges is a report from 2004 by the Los Angeles Times alleging that the late Justice Ruth Bader Ginsburg maintained a close relationship with the pro-abortion rights group National Organization for Women, which frequently had business before the high court.

He also listed a recent story by the Associated Press that raised questions concerning Justice Sonia Sotomayor's use of court staff to coordinate book sales and linked to a Fox News report raising questions about an investiture celebration for Justice Ketanji Brown Jackson that, the outlet reported, was funded by "undisclosed donors."

"None of these incidents has resulted in inquiries from this Committee," Rivkin wrote. "Where, as here, the scrutiny of an investigation is aimed at only one side of the political spectrum, it is a fair inference that politics is the motivating factor," he said.

"Whereas Mr. Leo is now the subject of a congressional inquiry, the many individuals and organizations who have facilitated travel for Democrat-appointed Justices, or exchanged gifts or personal hospitality with those Justices, are apparently immune from the Committee's attention," he said.