Story highlights "I have no excuse for what I have done," Radel says

He says he grew up with a mom who struggled with alcoholism

He pleads guilty to misdemeanor cocaine possession, gets a year of probation

Authorities say he bought 3.5 grams of cocaine from an undercover officer last month

U.S. Rep. Trey Radel said Wednesday night he will take a leave of absence for an unspecified time and donate his salary.

"I have no excuse for what I have done. I have let down our country," he said at a news conference.

Radel spoke to reporters after returning home to southwest Florida, hours after he pleaded guilty to misdemeanor cocaine possession.

The 37-year-old first-term Republican from Florida said he will enter an inpatient drug treatment program.

The cocaine possession charge came after authorities said he bought a small amount of cocaine in a sting in the nation's capital last month.

During the brief news conference, Radel said he "grew up with a mom who struggled with alcoholism."

"I don't want my son to struggle with that," he said.

The plea and sentence were part of a deal that Radel's attorney struck with federal prosecutors. He could have received a maximum sentence of 180 days imprisonment or a $1,000 fine, or both. Instead, he was placed on one year probation, and if it is "successfully completed," his guilty plea will be cleared from his record.

"Your honor, I apologize for what I've done," Radel told Judge Robert Tignor in court Wednesday. "I think in life I've hit a bottom where I realize I need help."

Radel said he is aggressively pursuing that help, with the support of friends and loved ones.

He was charged after he bought 3.5 grams of cocaine from an undercover police officer in Washington's Dupont Circle area on October 29, court documents say.

It was a sting that stemmed from a broader FBI/Drug Enforcement Administration investigation of a drug trafficking organization in the nation's capital, law enforcement officials told CNN on condition of anonymity.

The targets of the investigation are dealers and high-level people in the organization, not buyers and users. But a dealer who was arrested last month told federal agents that one of his customers was a congressman, one of the law enforcement officials said.

So, deciding they couldn't turn away from that, agents set up the sting, the law enforcement officials said.

Court documents say Radel gave the undercover officer $260 for the cocaine outside a restaurant, and the two got into the officer's vehicle, where the officer gave Radel the cocaine.

Federal agents approached Radel after he left the vehicle, and Radel dropped the cocaine to the street, according to court documents.

Radel then agreed to talk to the agents in his apartment, where he admitted he bought cocaine, according to the documents. "The defendant also retrieved and provided to the agents a vial of cocaine that he had in his apartment," the court documents say.

A tea party favorite, Radel was elected last year to represent southwest Florida's 19th Congressional District.

He is a former journalist and TV news anchor, having worked for WINK in Fort Myers, Florida. He also owned and eventually sold the Naples Journal newspaper, according to a biography on his congressional website.

Radel is married with a young son. He is active on Twitter, where he posts about touring the Capitol, listening to hip-hop, playing guitar and opposing Obamacare.

In a statement released by his office this week, Radel said he is "profoundly sorry to let down my family, particularly my wife and son, and the people of Southwest Florida."

"I struggle with the disease of alcoholism, and this led to an extremely irresponsible choice," he said. "As the father of a young son and a husband to a loving wife, I need to get help so I can be a better man for both of them.

"However, this unfortunate event does have a positive side. It offers me an opportunity to seek treatment and counseling. I know I have a problem and will do whatever is necessary to overcome it, hopefully setting an example for others struggling with this disease."

A spokesman for House Speaker John Boehner said in a prepared statement this week that members of Congress "should be held to the highest standards, and the alleged crime will be handled by the courts."

"Beyond that, this is between Rep. Radel, his family, and his constituents," Boehner's spokesman said.