Foley announced through his lawyer that he had been battling alcoholism and had checked into an unidentified rehabilitation facility for treatment over the weekend.
"I strongly believe that I am an alcoholic and have accepted the need for immediate treatment for alcoholism and other behavioral problems," Foley said. "I deeply regret and accept full responsibility for the harm I have caused."
Florida Republicans picked state Rep. Joe Negron to replace Foley as its candidate in the West Palm Beach district, which is largely Republican.
The FBI has opened a preliminary inquiry to determine whether Foley's sexually suggestive e-mails violated federal law. Foley abruptly quit Congress on Friday after reports surfaced that he'd sent sexually charged electronic messages to boys working as pages.
CBS News correspondent Bob Fuss reports that there is indication that members of the House leadership — all the way up to Speaker Dennis Hastert — were told about this more than a year ago. While they will admit they knew about the inappropriate e-mails, they say they never knew anything about sexually explicit ones.
"If it's proven that leaders in Congress did nothing, nothing to protect those children — those 15- and 16-year-olds — those members of Congress should resign their leadership positions," Rep. Sherrod Brown, D-Ohio, said, CBS News' Aleen Sirgany reports.
House Republicans, meanwhile, scrambled to limit political damage from the unfolding scandal following news that House leaders knew for months about Foley's inappropriate overtures toward the pages.
Hastert said Monday that no Republican leaders saw lurid Internet exchanges from Foley to pages and that he would have demanded the Florida Republican's expulsion if he had known about them.
"As a parent and speaker of the House, I am disgusted," Hastert, R-Ill., told reporters after holding a meeting at the Capitol in the wake of the disclosure of the e-mails in 2003 to a page, which led to Foley's resignation last Friday.
Hastert, in a letter to Attorney General Alberto Gonzales, asked the Justice Department to "conduct an investigation of Mr. Foley's conduct with current and former House pages."
Democrats are demanding that investigators determine whether Republican leaders tried to cover up Foley's actions for political reasons.
"The attorney general should open a full-scale investigation immediately," Senate Democratic leader Harry Reid of Nevada said in a statement, including whether GOP leaders "knew there was a problem and ignored it to preserve a congressional seat this election year."