WASHINGTON — Congressional ethics investigators said on Monday that they had gathered “substantial” evidence that Representative Ronny Jackson, Republican of Texas, improperly used campaign donations to pay for unlimited access for himself and his wife to a private dining club in Amarillo, Texas.

According to a report by the Office of Congressional Ethics, Mr. Jackson, who gained national attention as the White House doctor to former President Donald J. Trump and went on to win election to the House, is accused of using campaign money for the Amarillo Club, in violation of Federal Election Commission regulations prohibiting the use of campaign funds for such purposes.

“There is substantial reason to believe that Representative Jackson converted campaign funds from Texans for Ronny Jackson to personal use or that Representative Jackson’s campaign committee expended funds that were not attributable to bona fide campaign or political purposes,” the report said.

Its release came as the House Ethics Committee announced that it would continue investigating Mr. Jackson as well as another Republican aligned with Mr. Trump, Representative Alex X. Mooney of West Virginia, who a separate report accused of accepting a free or below-market trip to Aruba and using his staff to do personal errands.

The Amarillo Club is a private social club that includes fine dining, a wine program, a gym and banquet and meeting room spaces. It is affiliated with a national network of private golf and country clubs, and affords its members privileges at golf courses throughout the United States.

Mr. Jackson refused to cooperate with the investigation, and his campaign’s treasurer and accounting firm refused to provide documents to investigators. His lawyer said on Monday that the congressman had planned to use the club membership for campaign-related events.

The report, which included detailed transaction information about charges made by Mr. Jackson at the club, showed that in October 2020, Mr. Jackson used $649.50 in campaign funds to join the club, and his campaign had since paid $175.37 monthly in dues, totaling $1,929.07.

Between October 2020 and September 2021, Mr. Jackson’s campaign paid $5,907.13 to the club for dues, fees, meals and other services.

Mr. Jackson’s membership allowed him and his wife unlimited use of the club’s dining rooms, gym, banquet and meeting rooms, as well as access to club events and other benefits, investigators said.

In a letter to the ethics office, Justin R. Clark, Mr. Jackson’s lawyer, said the campaign had purchased the Amarillo Club membership “primarily to use such meeting space for internal and external meetings, including but not limited to fund-raising events, for campaign purposes due to the proximity of the Amarillo Club to the campaign’s office.”

Mr. Clark said that the campaign “has not utilized the meeting space as frequently as originally anticipated,” but that the “campaign purpose of the expenditure nevertheless exists.”

“Neither Congressman Jackson nor any members of his family have utilized any benefits of the Amarillo Club other than dining and meeting spaces for campaign purposes,” Mr. Clark wrote.

However, investigators said Mr. Clark “declined to provide any documents or testimony in support of this position.”

The Office of Congressional Ethics is an independent watchdog that refers cases to the Ethics Committee, which can determine punishments. A second report the office released on Monday found there was “substantial reason to believe” that Mr. Mooney had also acted improperly.

Investigators found that Mr. Mooney and his family enjoyed a vacation in early March 2021 at the Ritz-Carlton in Aruba, paid for by HSP Direct, a company to which Mr. Mooney has significant financial and personal ties. Congressional staff resources were devoted to arranging travel for the Mooneys, investigators said.

HSP Direct’s payment for the vacation, totaling at least $10,803, probably constitutes an impermissible gift under House rules, investigators wrote.

Additionally, investigators found that Mr. Mooney “may have used official resources, including staff time, for campaign work and personal errands” and may have “withheld, concealed or falsified information” during the ethics investigation.

Seven former and current employees described frequent requests to complete unofficial tasks — ranging from babysitting and car repairs for a personal vehicle to assisting with personal finances and businesses — solely for the personal benefit of Mr. Mooney and his family.

When the Mooneys were unable to watch their dog Skipper, for instance, they asked a former aide to drive Skipper from their home in Charles Town, W.Va., to a relative’s home in Bethesda, Md.

Multiple former staff members also were expected to gather Mr. Mooney’s dirty clothes from various places in the official office and have them taken to the dry cleaners in the Longworth House Office Building, investigators said. Mr. Mooney placed his worn shirts and suits on one staff member’s office chair, which, she said, meant he wanted the clothes to go to the laundry.

Mr. Mooney also asked an aide to take a shirt and a towel home with her to wash in her washing machine, the report stated.

A spokesman for Mr. Mooney did not immediately respond to a request for comment.

The reports came as the Ethics Committee also announced it was beginning an investigation into Representative Madison Cawthorn, Republican of North Carolina, to determine whether he improperly promoted a cryptocurrency in which he had an undisclosed financial interest and engaged in an improper relationship with someone on his congressional staff.

Mr. Cawthorn lost his primary race last week after a string of unflattering revelations. In a statement on Monday, Blake Harp, his chief of staff, said, “We welcome the opportunity to prove that Congressman Cawthorn committed no wrongdoing and that he was falsely accused by partisan adversaries for political gain.”