In this September 12, 2019 file photo, U.S. Rep. Chris Collins, R-N.Y., speaks to reporters as he leaves the courthouse after a pretrial hearing in his insider-trading case, in New York.
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(CNN) —  

New York Rep. Chris Collins submitted his resignation from Congress on Monday, one day ahead of his expected guilty plea to federal charges in an insider trading case, according to the House Speaker’s office, court documents and a person familiar with the matter.

Collins sent a letter of resignation Monday to the office of House Speaker Nancy Pelosi, according to her spokesman, who said Collins’s resignation will be effective Tuesday.

The first sitting congressman to back President Donald Trump’s bid for the White House, Collins was reelected to office several months after he was originally indicted.

He faced reelection in 2020. While a guilty plea wouldn’t by law have caused him to immediately lose his congressional seat unless the House of Representatives had expelled him, a step that would have required a full vote of the House, it would have made for a challenging reelection bid.

The Republican congressman narrowly retained his seat in western New York in 2018, beating his Democratic opponent by less than half a percentage point.

It’s not clear if Collins, who was charged by the Manhattan US Attorney’s office, is set to plead guilty to precisely the same set of charges contained in the indictment against him, which was originally filed in August 2018 and was revised last month.

Collins’ attorneys, Jonathan New and Jonathan Barr, didn’t immediately respond to a request for comment.

Collins’ co-defendants – his son and another man – are also set to change their pleas on Thursday, according to court filings. All three had been set to face trial in February 2020.

Collins and his co-defendants had pleaded not guilty twice in the case, once after the original set of charges in August 2018 and a second time – to the revised charges contained in what’s known as a superseding indictment – earlier this month.

Federal prosecutors in the Manhattan US Attorney’s office allege that the defendants acted on non-public information about the results of a drug trial, which was then used to trade on the stock of the pharmaceutical company, Innate Immunotherapeutics Limited, of which Collins was a board member.

The indictment didn’t allege that Collins himself traded on information about the failed results of a drug trial, but that he passed the information to his son so that his son could execute trades.

That chain of information allowed the three defendants to avoid more than $768,000 in losses they would have incurred if they had traded the stock after the drug trial results became public, prosecutors alleged.

The indictment also highlighted a memorable claim: that Collins was attending the annual congressional picnic at the White House on June 22, 2017, when he received the information about the failed drug trial from the company’s CEO and then repeatedly dialed his son.

Speaking outside the courthouse after his second not guilty plea, Collins said he believed he would succeed at trial. “I look forward to being exonerated in due course,” he said.

This story has been updated with additional developments Monday.

CNN’s Clare Foran contributed to this report.