Rep. Alcee Hastings, D-Fla., listens to students speak about their experiences with gun violence during the The Gun Violence Prevention Task Force panel Wednesday afternoon May 23, 2018.

Rep. Alcee Hastings, D-Fla., who is facing a House ethics probe for engaging in a relationship with a subordinate staff member, has been paying his girlfriend the highest salary among his staffers for nearly a decade, according to House disbursement records.

Patricia Williams, Hastings' girlfriend of at least 25 years, has been a congressional staffer in his office for 27 years, Hastings has said in press reports. Library of Congress records show that Williams was an aide to Hastings from 2002 to 2006, before transitioning into her role as deputy district director, which she held from 2006 to 2018. She is now the district director.

Hastings, who makes $174,000 a year as a congressman, paid Williams $168,411 in 2018, the maximum congressional staff salary. That was more than Hastings' chief of staff at the time, Arthur W. Kennedy, made despite being Williams' boss. Kennedy received a salary of $166,978 that year.

Hastings began paying Williams more than Kennedy starting in 2011.

Hastings told the Miami Herald on Thursday that Williams' high salary derives from her seniority. "One of the things, and I will say this to you, that I do deeply resent is the comparison, for example, of her salary after 27 years to other members' staffers who have worked five years or six years," he said.

The Palm Beach Post reported that Williams has earned nearly $3 million since 2000.

Hastings also employs Williams' daughter in his Broward County, Florida, office, according to the Washington Free Beacon.

Katrina Martell, Hastings' communications director, did not respond to numerous emails and phone calls from CNBC.

Traditionally, the chief of staff is the highest-paid congressional staffer and reports directly to the member of Congress.

Kennedy was Hastings' chief of staff for 25 years, from 1994 to 2019, according to the Library of Congress. A congressional staff listing indicates Lale Morrison is Hastings' current chief of staff. House disbursement records indicate that in 2018, Morrison earned about $92,665 in her role. Library of Congress records indicate that Morrison has been in her role since February 2009 and appears to have shared the chief of staff position with Kennedy.

In the last quarter of 2018, salaries ranged from about $8,900 to $41,000 for deputy district directors, according to House disbursement records, which comes out to about $35,600 to $164,000 annually.

Nancy Bull, deputy district director for former Rep. Bill Shuster, R-Pa., during the quarter, was the only one in this role whose annual salary matched that of Williams'.

About 90% of all deputy district directors received less than $29,000 that quarter, meaning they would earn less than $116,000 annually.

Hastings told the Sun Sentinel that Williams advises him on immigration issues and has "worked with me from Day One ... It would be one thing if she didn't work, but she's working today, and she has continued to work."

Prior to Hastings' congressional career, when he was a federal judge, Williams, an attorney, represented him in a 1983 bribery trial that resulted in his removal from the bench.

Read more: House ethics committee reveals separate probes into two congressmen on the same day

The House Ethics Committee revealed on Nov. 14 that a probe into the relationship between Hastings and Williams has been going on since May. The 83-year-old congressman said in a statement on the same day that he's "cooperated with the Committee since May 14, 2019. As they continue to conduct their work, I stand ready to fully cooperate with their inquiry."

The House passed a rule last year that prohibits its members from having a relationship with a staffer, in the wake of the #MeToo movement.

Former Rep. Katie Hill, D-Calif., announced her resignation on Oct. 27 amid an ethics probe into an alleged relationship with a subordinate congressional staffer. Hill admitted to a relationship with a female campaign staffer but denied having a relationship with a male congressional staffer.

— Graphic by CNBC's Nate Rattner

Correction: An earlier version misidentified whom Nancy Bull worked for. She was deputy district director for former Rep. Bill Shuster, R-Pa.