The Senate Ethics Committee said Thursday that Democratic Sen. Robert Menendez, who avoided conviction in a federal corruption trial last year, violated federal law and Senate rules in accepting unreported gifts from a friend and political ally.

In a four-page “letter of admonition,” the panel’s three Republicans and three Democrats ordered Menendez to pay back the gifts he received from Salomon Melgen, a Florida eye doctor, and said he is “hereby severely admonished.”

“Your assistance to Dr. Melgen under these circumstances demonstrated poor judgment, and it risked undermining the public’s confidence in the Senate,” the letter reads. “As such, your actions reflected discredit upon the Senate.”

The Ethics Committee findings complete a winding, six-year saga in which the New Jersey senator was accused of corrupt dealings to benefit a political donor, Melgen, only to see his prosecution end in a mistrial in November after 10 weeks in court.

As long as Menendez repays the gifts, he is unlikely to face further official sanctions. But the stern reprimand stands to cloud his pending campaign for a third Senate term, though he retains an advantage as a Democratic incumbent in a liberal-leaning state.

Republicans pounced immediately on the findings. His likely GOP opponent called on Menendez to resign, and a spokesman for the National Republican Senatorial Committee called him “a crook and an embarrassment.”

Marc Elias, a lawyer representing Menendez, said Thursday that the letter contained findings that were “contradicted by the presiding judge and rejected by the jury” and said the trial “clearly demonstrated there was no violation of any law.”

“With the Ethics Committee process now concluded, Sen. Menendez looks forward to continuing to serve the people of New Jersey,” Elias added.

After the mistrial, most jurors told reporters that they believed Menendez had not committed the crimes federal prosecutors alleged and a grand jury had charged him with. On Jan. 31, a judge dismissed all charges against Menendez after prosecutors decided not to pursue a second trial.

According to the Ethics Committee letter, the panel relied heavily on the evidence and testimony presented at Menendez’s trial, which the panel said it “did not need to independently duplicate.”

It was not immediately clear how much Menendez might have to repay. In 2013, after media reports detailing gifts he had taken from Melgen, he repaid Melgen $58,500 for a pair of 2010 trips aboard Melgen’s private plane — an expenditure that, according to personal financial disclosures, cut deeply into Menendez’s modest personal wealth.

But prosecutors detailed more than a dozen flights on private jets paid for by Melgen, and other significant gifts, including use of Melgen’s Caribbean villa, a Paris hotel stay and golf outings. The Paris stay, paid for with Melgen’s American Express rewards points, is alone valued at nearly $5,000. Their indictment also characterizes as a gift $40,000 in donations that Melgen and members of his family made to a Menendez legal defense fund in 2011 and 2012.

Tricia Enright, a Menendez spokeswoman, said the senator made an additional repayment of $46,975 in 2014 to cover other gifts and separately repaid the hotel bill. Elias said that after Menendez made the initial repayments, Menendez “halted further remedial action” at the Ethics Committee’s request, including updating his financial disclosure forms.

Since prosecutors abandoned their case, Menendez has moved to reassert his place in the Senate, taking back the ranking Democrat post on the Foreign Relations Committee that he temporarily vacated during his prosecution. As the most senior senator of Latino heritage, he has also reemerged as a strong advocate for immigration reform, including protections for the young immigrants covered under the Deferred Action for Childhood Arrivals program.

Before Thursday’s letter, Menendez appeared to be well-positioned in his race against a relatively unknown Republican challenger, pharmaceutical executive Bob Hugin. A Quinnipiac University poll published last month gave Menendez a 17-point lead and found that fewer than 40 percent of voters believed he had engaged in “serious wrongdoing” in office.

Federal prosecutors, after media reports of Menendez’s actions to help Melgen settle business disputes with the federal government, secured indictments on 14 charges, including bribery. They alleged Menendez used his office to help Melgen after the doctor gave gifts to Menendez, including the private-jet flights and stays at Melgen’s Dominican Republic villa. Melgen also gave campaign donations that were funneled to Menendez’s 2012 campaign.

But in court, Menendez successfully argued that there was no illegal quid pro quo, that Melgen was a bona fide friend and that the gifts he received and the favors he did for him were unconnected.

The panel concluded that Menendez’s efforts to aid Melgen “went well beyond Senate norms” — including arranging a 2012 meeting with then-Senate Majority Leader Harry M. Reid (D-Nev.) and Health and Human Services Secretary Kathleen Sebelius.

The letter also singled out the private-jet flights for particular scrutiny, noting that Menendez — just days after joining the Senate — voted for legislation requiring senators to pay fair-market value for any private-plane travel.

While it is rare for the Senate Ethics Committee to investigate a sitting senator, it has broad power to investigate lawmakers’ conduct, both while in office and beforehand, and it has taken serious action on several occasions.

Since the committee’s establishment in 1964, it has twice recommended that senators be expelled from the body: Sen. Harrison A. Williams Jr. (D-N.J.) in 1982 after he was convicted on corruption charges; and Sen. Bob Packwood (R-Ore.) in 1995 after a lengthy investigation found evidence of rampant sexual misconduct. Both senators resigned before the Senate could vote on their expulsion. In that time, three other senators have been recommended for censure, an official rebuke that falls short of expulsion.

Since 2002, the panel has issued letters of admonition to six senators. One was delivered to another New Jersey Democrat, Robert G. Torricelli, who was ordered in 2002 to repay a political donor after accepting a TV, CD player and three pairs of earrings for less than their full-market value. Torricelli subsequently abandoned his campaign for a second term.