The House Ethics Committee on Thursday found “substantial evidence” that Representative George Santos violated federal law, setting the stage for another push to expel the embattled first-term Republican from New York and prompting him to declare that he would not seek re-election.

House investigators found widespread evidence that Mr. Santos used campaign funds for personal purposes, defrauded donors and filed false or incomplete campaign finance and financial disclosure reports, according to a scathing 56-page report released on Thursday.

Mr. Santos “sought to fraudulently exploit every aspect of his House candidacy for his own personal financial profit,” the report said, adding that he sustained his campaign “through a constant series of lies to his constituents, donors, and staff about his background and experience.”

The committee voted unanimously to refer its findings to the Department of Justice, saying that Mr. Santos’s conduct “warrants public condemnation, is beneath the dignity of the office, and has brought severe discredit upon the House.”

And while the panel refrained from recommending any punitive measures, its chairman, Representative Michael Guest, Republican of Mississippi, separately announced that he would introduce a new motion to remove Mr. Santos from office as soon as Friday.

“Most of us have never seen anything like this — this extensive, this brazen, and this bold,” said Representative Glenn F. Ivey, Democrat of Maryland, who sits on the Ethics Committee.

In the hours after the report was released, a handful of Democrats and Republicans who had previously voted to protect Mr. Santos indicated they now supported his removal.

Mr. Santos, 35, a Republican representing parts of Long Island and Queens, already faces a 23-count federal indictment that includes accusations that he stole from his donors and falsified campaign filings.

He successfully flipped a Democratic district last November, but his victory was quickly overshadowed by reports from The New York Times and others revealing that he had fabricated much of his life story.

Mr. Santos, who did not respond to a request for comment, has resisted calls for his resignation, and has pleaded not guilty. But shortly after the Ethics Committee report was released, Mr. Santos announced on X, the website formerly known as Twitter, that he would not seek re-election in 2024.

He went on to take issue with the panel’s findings, however, writing: “If there was a single ounce of ETHICS in the “Ethics committee,” they would have not released this biased report.”

The report, which was the product of nearly nine months of investigation, was damning and comprehensive, and accompanied by hundreds of pages of supporting documents.

Investigators reported finding evidence of widespread malfeasance throughout Mr. Santos’s congressional campaigns, from fictitious loans to brazen grift.

Mr. Santos famously presented himself on the campaign trail as a wealthy man, with stints at big banks like Goldman Sachs, an advanced degree and a family fortune that could help him personally loan his campaigns thousands of dollars.

Congressional investigators found the opposite was true.

During his first, unsuccessful run for Congress in 2020, for example, Mr. Santos purported to loan his campaign $81,250 in several installments reflected on F.E.C. reports. In actuality, he only ever transferred $3,500.

When the campaign ended, however, he repaid himself a large portion of the original, fictitious loans anyway, and made $27,700 in profit.

The figures ballooned in 2022, when Mr. Santos reported lending his campaign more than $700,000. Those loans were eventually made, the report says — not when they were first reported in the spring, but in a series of transfers in the fall from Mr. Santos’s personal and company accounts.

Congressional investigators traced the funds to services he performed for outside clients, but noted that the timing and other circumstances of the payments raised “serious questions” about whether they were unlawful campaign contributions.

The report also detailed a multitude of errors and omissions in Mr. Santos’s financials, some — like more than three dozen expenditures between $199 and $200, pennies below the threshold where receipts are required — that investigators say suggest fraud.

Elsewhere, the report detailed how Mr. Santos moved large sums between a New York PAC, a Florida LLC and his personal account, at one point noting that he had withdrawn a total of $240,000 that has not been accounted for.

And investigators charge that Mr. Santos “blatantly” stole from his campaign, detailing travel and hotel stays in Las Vegas that corresponded to the time when he told staff members he was on his honeymoon, and thousands of dollars spent at spas. At least two payments were described as being for Botox.

Some of the most salacious details of the report concern a company called RedStone Strategies, which Mr. Santos used to raise money without being constrained by campaign contribution limits.

Investigators found that Mr. Santos transferred at least $200,000 to himself from RedStone through numerous transactions in 2022, some of which was used to pay off personal credit cards and make purchases at Hermes, Sephora and OnlyFans, a website known for its adult content.

The findings appeared to undermine Mr. Santos’s underlying criminal defense strategy. The congressman has repeatedly denied involvement in his campaign’s finances, saying that his treasurer had “gone rogue.”

But investigators found that Mr. Santos was “heavily involved” in financial matters, noting that he received weekly finance reports and invoices and had login credentials to campaign bank accounts.

Mr. Santos’s treasurer, Nancy Marks, pleaded guilty to federal charges last month, becoming the first person associated with his campaign to do so. The second, a campaign aide named Samuel Miele, pleaded guilty earlier this week.

The committee was not able to substantiate accusations that Mr. Santos was guilty of sexual harassment. The allegations were made last February by a prospective aide who accused Mr. Santos of coming on to him, then firing him after his advances were rebuffed. The Santos campaign said that it had fired the aide after learning of wiretapping charges he faced in Ohio.

Earlier this month, a bloc of first-term Republicans representing moderate districts in Mr. Santos’s home state of New York forced a vote on his expulsion from Congress. That effort failed decisively, with many House members saying they were leery of setting a precedent for removal without a finding from a court or the Ethics Committee.

Those critics, including Anthony D’Esposito and Nick LaLota, two Long Island Republicans, have wasted little time in using the ethics investigation to push their colleagues on both sides of the aisle to reconsider Mr. Santos’s expulsion.

The Ethics Committee has described the Santos investigation, which came after a series of complaints from both members of Congress and the public, as “a priority for the investigative team” that utilized a “significant amount of the committee’s resources.”

The panel contacted roughly 40 witnesses, reviewed more than 170,000 pages of documents, and authorized 37 subpoenas.

The probe covered a range of alleged criminal and ethical violations, including accusations that the congressman violated conflict of interest laws while conducting business for his firm, the Devolder Organization, and failed to properly file financial disclosures.

Some of that conduct is also being taken up by federal prosecutors, who have charged Mr. Santos with a variety of crimes involving his personal and campaign finances.

Mr. Santos’s re-election campaign had already faced an uphill battle. Even before he announced in April that he would run for re-election, state and county party officials said they would not support his efforts. His fund-raising totals lagged behind potential challengers.

Last month, Thomas R. Suozzi, the Long Island Democrat whose failed bid for governor in 2022 may have helped clear the way for Mr. Santos to get to Congress, announced that he would run for his old seat, in hopes of returning the swing district to Democratic control.

Luke Broadwater contributed reporting from Washington and Michael Gold from New York.