Sen. Ed Markey, D-Mass., found himself apologizing Tuesday after a top campaign aide retweeted a message about "mental health issues" in the family of Rep. Joe Kennedy, a potenial challenger for Markey's Senate seat.

The retweet -- which Markey termed "unacceptable" -- came just three weeks after Kennedy's young cousin, Saoirse Kennedy Hill, 22, died of an apparent drug overdose.

The Markey aide, Paul Tencher, retweeted a post by attorney James S. Henry, who asserted that Markey "is a great Senator," while Kennedy "should focus on his family's considerable mental health issues" -- seeming to reference the cousin's death.

JOE KENNEDY III MULLING DEM PRIMARY CHALLENGE FOR MASSACHUSETTS SENATE SEAT

After Markey learned of the retweet, he issued the following statement:

"To show such insensitivity for those experiencing mental health issues is extremely offensive to me. This action by a member of my campaign is unacceptable,” Markey said, according to the Washington Examiner. “It in no way reflects my commitment to the issue of mental health treatment and the eradication of stigma that so many families suffer. I deeply apologize to Congressman Kennedy and his entire family.”

"To show such insensitivity for those experiencing mental health issues is extremely offensive to me. ... I deeply apologize to Congressman Kennedy and his entire family.” — Sen. Ed Markey, D-Mass.

Markey added that he had spoken with the congressman directly about the matter.

The 38-year-old Kennedy -- a grandson of Robert F. Kennedy -- is considering a primary challenge in 2020 against the 73-year-old Markey, the New York Times reported Saturday. The paper cited an unnamed Democratic official who said Kennedy would decide on a candidacy in the coming weeks.

Kennedy previously stated he planned to seek a fifth term in the House, representing Massachusetts’ 4th Congressional District, Boston.com reported.

Saoirse Kennedy Hill died Aug. 1 at the Kennedy compound in Hyannis Port, Mass. In 2016, she wrote a column in the Deerfield Scroll, a student-run newspaper, discussing her quiet mental health struggles. Her grandfather was the late Sen. Robert F. Kennedy.

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On Sunday, Tencher apologized for disseminating tweet amid backlash. The tweet’s author, attorney Henry, deleted the original message, the Washington Examiner reported.

"I absolutely take it back. It was a mistake. My apologies," Tencher said on Twitter. "That tweet was despicable and abhorrent. This person should be banned from twitter."

Fox News' Alex Pappas contributed to this report.