It was an opportunity that Representative Chris Collins, a Republican from Western New York, believed was too good to pass up. He was so confident of his investment in an Australian drug company that he hawked it to his children, his staffers, even other members of Congress.

“One of the things was, ‘Who in Congress have you talked with about Innate?’” Mr. Collins said of the company, Innate Immunotherapeutics, in a 2017 interview. “The bigger question would be: Who haven’t I talked to?”

A version of that question became Mr. Collins’s undoing. On Tuesday, he pleaded guilty to conspiracy to commit securities fraud and lying to federal investigators, after admitting passing private information about Innate to his son to help him avoid financial losses. Mr. Collins faces a prison sentence of up to 10 years; his son and the father of his son’s fiancée are expected to plead guilty to related charges later this week.

“I regret these actions beyond anything,” Mr. Collins, 69, told the judge, Vernon S. Broderick, in Federal District Court in Manhattan.

In his speech, Mr. Collins apologized to his son, “who I’ve spent my life supporting,” and to his constituents.

“I have tried to be a model citizen to them,” he said. “The actions I took are anything but.”

The charges stemmed from Mr. Collins’s significant financial ties to Innate; he owned nearly 17 percent of the company’s total stock, making him one of its largest shareholders. He was also a member of its board of directors.

According to prosecutors, Mr. Collins — who rose to prominence as the first sitting member of Congress to endorse Donald Trump in the 2016 election — was at a congressional picnic in June 2017 when he learned, through a private company email, that Innate had failed a critical trial for a drug to cure multiple sclerosis. The drug, prosecutors said, was the company’s only significant product.

Immediately, Mr. Collins called his son, Cameron Collins, to encourage him to sell his shares before the test results became public.

“I was in a very emotional state,” Mr. Collins said Tuesday of that moment, adding that he knew the news “would have devastating effect” on Innate’s stock price.

The congressman called his son several times in quick succession before they finally connected on the seventh try, according to call logs.

The tip helped Cameron Collins avoid more than $570,000 in losses when the company’s stock price later plummeted, prosecutors said.

“Collins admitted to, among other things, illegally tipping his son while standing on the White House lawn,” the United States attorney in Manhattan, Geoffrey S. Berman, said in a news conference shortly after the plea.

Cameron Collins, as well as Stephen Zarsky, the father of Cameron Collins’s fiancée, are expected to plead guilty on Thursday. Cameron Collins had advised Mr. Zarsky and Mr. Zarsky’s daughter to sell their shares, prosecutors said, and Mr. Zarsky also passed the tip to others.

The younger Mr. Collins’s fiancée was not charged.

The congressman himself, who was ranked as one of the wealthiest members of Congress, was not accused of making any trades. But just the act of passing along information can, under some circumstances, constitute an insider-trading violation.

Mr. Collins is scheduled to be sentenced on Jan. 17. Though the maximum sentence for the two charges is 10 years, prosecutors and defense lawyers agreed to seek a sentence between 46 and 57 months.

Mr. Collins’s plea capped a political career that had been remarkable to that point for its resilience. Last year, Mr. Collins, who was first elected in 2012, narrowly won re-election even after being indicted on the insider trading charges to which he pleaded guilty on Tuesday. Until this week, he had offered hints that he planned to run again, despite pleas from fellow Republicans to step aside.

That all changed on Monday, when lawyers for Mr. Collins — who was staring down a trial scheduled for February, before a judge who had denied many of his pretrial motions — announced in a court filing that the congressman planned to change his not guilty plea.

Within hours, Mr. Collins resigned his seat.

A lawyer for Mr. Collins did not respond to questions about what prompted the change in plea.

Congressional investigators had raised ethical questions before about Mr. Collins’s relationship to Innate. Mr. Collins’s 2017 interview about the company was part of a probe by the Office of Congressional Ethics, which later concluded that Mr. Collins had potentially violated federal law as well as House ethics rules.

In that interview, Mr. Collins said that his son and daughter owned shares in Innate, as did “most” members of his congressional staff. At least five Republican lawmakers also owned Innate stock at the time of the failed drug trial.

Last August, after Mr. Collins was indicted, then House Speaker Paul Ryan stripped Mr. Collins of his seat on the Energy and Commerce Committee, which oversees health care companies. Mr. Ryan also asked the House Ethics Committee to investigate the allegations.

Mr. Collins’s resignation jolted the already-brewing race to replace him. Last year, campaigning under the shadow of the indictment, Mr. Collins defeated his Democratic challenger, Nate McMurray, by less than half a percentage point — even though his district voted for Mr. Trump by a margin of nearly 25 points.

But with the taint of Mr. Collins’s legal troubles gone, political pundits predicted that the district would remain safely Republican. Soon after Mr. Collins’s resignation, two Republican candidates, State Senators Chris Jacobs and Robert Ortt, each declared their commitment to advancing Mr. Trump’s agenda.

Mr. McMurray, for his part, said in a statement that Mr. Collins as well as Republican Party insiders had wronged residents of the district, which falls between Buffalo and Rochester.

Gov. Andrew M. Cuomo, a Democrat, must call a special election to fill Mr. Collins’s seat before next November’s general election, though the timing of that election is flexible. In a radio interview on Tuesday, Mr. Cuomo said he wanted to call it “sooner rather than later.”

Mr. Collins is not the first elected official to face federal charges intertwined with a son’s behavior. Last year, the former majority leader of the New York State Senate, Dean G. Skelos, was convicted of public corruption after prosecutors said he pressured business executives to pay his son about $300,000, through various no-show or low-show jobs. His son, Adam, was also convicted.

Mr. Skelos’s deputy, State Senator Thomas W. Libous, was separately accused of lying to federal authorities about the circumstances of his own son’s employment. He was convicted but the conviction was later vacated after his death; his son, Matthew Libous, was convicted of federal tax charges.

After the hearing, Mr. Collins, flanked by his lawyers, exited the courthouse directly into a black sport utility vehicle idling nearby. He ignored shouted questions from reporters.