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."
In related developments:
was "outraged and disgusted with Congressman Mark Foley's actions."
Foley's district is heavily Republican, but now may be won by a Democrat. Republicans are struggling to maintain their House majority in the upcoming election.
The Washington Post reported that Patrick McDonald, a former House page, said that he had learned that three or four pages in his 2001-2002 class had received inappropriate e-mails from Foley.
McDonald, 21, said he had seen Foley e-mails sent to another page during a 2003 page reunion and recalled saying, "If this gets out, it will destroy him."
FBI cyber-sleuths are looking into the text of some of Foley's messages, checking to see how many e-mails and instant electronic messages were sent and how many computers were used, according to a law enforcement official, who spoke on condition of anonymity because of the ongoing investigation.
The FBI also was trying to determine if any of the teenagers who received messages are willing to cooperate with the investigation, the official said.
Ironically, Foley, who is 52 and single, could be found to have violated a law that he helped to write as co-chairman of the Congressional Missing and Exploited Children's Caucus.
"Republican leaders have admitted to knowing about Mr. Foley's outrageous behavior for six months to a year, and they chose to cover it up rather than to protect these children," said House Democratic leader Nancy Pelosi of California.
Rep. Thomas Reynolds of New York, head of the House Republican election effort, said he told Hastert months ago about the allegations involving a 16-year-old boy from Louisiana.
Hastert acknowledged that his staff had been made aware of concerns about what they termed "over-friendly" e-mails Foley had sent to the teenager — including one requesting his picture — in the fall of 2005, and that they referred the matter to the House clerk.
But Hastert said those e-mails were not viewed as "sexual in nature" and that he was not aware of "a different set of communications which were sexually explicit ... which Mr. Foley reportedly sent another former page or pages."
Hastert asked the Justice Department to investigate "anyone who had specific knowledge of the content of any sexually explicit communications between Mr. Foley and any former or current House pages and what actions such individuals took, if any, to provide them to law enforcement."
A Pelosi spokeswoman, Jennifer Crider, said that Hastert "seems more concerned by who revealed the Republican leadership cover-up of Mr. Foley's Internet stalking of an underage child than he was about ensuring the children entrusted to the House were protected."
Congressional pages, a staple of Washington politics since the 1820s, are high school students who serve as gofers in the House and Senate.