WASHINGTON -- In 27 years as a congressman and senator representing Idaho, Larry Craig built a long legislative record.

He played a key role in enacting a law that shields gun-makers and sellers from lawsuits over misuse of their weapons. He helped broker a deal that led to legislation aimed at preventing forest fires. And he steered millions of dollars to his state for projects.

But just four days, 19 hours and 42 minutes after the first report of his arrest in a sex sting operation was posted on the Internet, his political career came to an end. He faces the prospect of being remembered, not for his legislative record, but for his police record.

Yesterday, Mr. Craig announced "with sadness and deep regret" that he will resign from the Senate at the end of the month. "What is best for Idaho has always been the focus of my efforts, and it is no different today," he said.

Mr. Craig explained that he hopes to withdraw his guilty plea for disorderly conduct and said that would be an "unwarranted and unfair distraction of my job and for my Senate colleagues."

Idaho Gov. C.L. "Butch" Otter, who joined his former congressional colleague at a Boise, Idaho, news conference, is expected to appoint another Republican, most likely Lt. Gov. James E. Risch, to serve the remainder of Mr. Craig's term, which ends in January 2009. Mr. Craig, appearing with his wife and two of his three children before perhaps the largest media gathering he has ever faced, again apologized for the events that abruptly and ignominiously stained his legacy as one of Congress' leading voices on Western issues.

"To Idahoans I represent, to my staff, my Senate colleagues, but most importantly, to my wife and my family, I apologize for what I have caused. I am deeply sorry," he said.

It was not clear whether Mr. Craig would fly back to Washington, D.C., to join his Senate colleagues when they return Tuesday from their month-long summer recess.

Mr. Craig, 62, who served 17 years as a senator after 10 years in the House, went into a political free fall after his guilty plea was disclosed Monday.

In June, Mr. Craig was arrested at Minneapolis-St. Paul International Airport by an undercover officer investigating complaints that men were soliciting sex in a restroom. The officer said Mr. Craig tapped his foot and slid his hand under a stall divider, which the officer said indicated a desire for sex. Mr. Craig paid $575 in fines and fees in early August and was given one year's probation.

At a news conference Tuesday in Boise, the state capital , Mr. Craig denied doing anything inappropriate, insisted repeatedly that he was not gay, and said he regretted pleading guilty.

Mr. Craig's downfall was swift, even by Washington standards. But his guilty plea and the extensive media coverage of the incident became a headache of migraine proportions for Republicans at a time when the party is gearing up for the 2008 congressional campaign.

The party has been unable to escape the taint of scandal that contributed to its losses in the last elections. Alaska Sen. Ted Stevens, the longest-serving Republican senator, is under investigation; and Louisiana Sen. David Vitter recently apologized for a "very serious sin" after his phone number turned up in the records of an alleged madam.

GOP colleagues in Idaho and Washington, D.C. , leaned heavily on Mr. Craig to give up the seat. Two Republican senators called on him to resign, others denounced his alleged behavior. The party's leadership asked for an investigation by the ethics committee and stripped Mr. Craig of his committee positions.

Last week, the National Republican Committee considered issuing a statement calling on Mr. Craig to resign but held off at the request of Idaho GOP officials who wanted to meet with the senator, said a Republican aide who spoke on condition that he not be named because he was not authorized to speak about the matter.

Idaho remains a very red state, and a Republican is heavily favored to win the seat in November 2008.

Republican leaders expressed relief with Mr. Craig's decision.

A number of prominent Republicans had urged him to resign or remained conspicuously silent. But yesterday, they took a more sympathetic tone.

President Bush telephoned Mr. Craig to tell him he knew it was difficult decision and to wish him well, said White House spokesman Scott Stanzel, who added that Mr. Craig "made the right decision for himself, his family, his constituents and the U.S. Senate."

Senate Minority Leader Mitch McConnell, R-Ky., said in a short statement that Mr. Craig made the right choice.

Mr. Craig could qualify for a $98,000-a-year congressional pension, assuming he joined the system upon entering the House in 1981, according to a estimate made by the National Taxpayers Union.

Earlier this year, Mr. Craig voted for a Senate measure that would deny pensions to lawmakers convicted of serious crimes, such as bribery, conspiracy and perjury.