"It's clear to me that a vigorous debate on the issues most likely could not take place if I remain in the race," added the statement, a copy of which was obtained by The Associated Press.
"What would take place, rather, is a brutal, scorched-earth campaign — the kind of campaign that has turned off so many voters, the kind of politics I refuse to play."
Illinois GOP leaders would have to select another candidate in the event of a withdrawal. Ryan's replacement would become an instant underdog in a campaign against Democratic State Sen. Barack Obama.
Ryan conducted an overnight poll to gauge his support in the wake of the allegations made by his ex-wife in divorce records unsealed earlier this week. Aides said in advance his only options were to withdraw or to redouble his campaign efforts with a massive infusion of money from his personal wealth.
After reviewing the polling results, Ryan's advisers told the candidate Friday morning that he could survive the scandal but only after an extremely negative and expensive response. "I won't do that," Ryan replied, according to a participant in the meeting. "That's not me."
The internal polling had Ryan trailing Obama 20 to 25 percent, the official said. That figure didn't worry aides as much as results showing that conservatives were abandoning Ryan. They concluded that the only way to get Ryan's base back would be to go negative immediately on Obama and not let up.
Illinois Republican Party leaders convened a teleconference in the hours before Ryan made his announcement, although it wasn't clear whether they had yet turned to discussion of who might replace Ryan some four months before the November election.
Several names immediately surfaced, including state Sen. Steve Rauschenberger and dairy owner Jim Oberweis, both of whom lost to Ryan in the primary, and former state Board of Education chairman Ron Gidwitz.
Additionally, other GOP officials floated the name of Patrick Fitzgerald, a U.S. attorney in Illinois. Fitzgerald has achieved national prominence recently with his appointment to head a probe into the leak of a CIA operative's name to a journalist. He interviewed President Bush earlier this week for more than an hour as part of the effort.
Polls have shown Ryan trailing Obama from the start of their race. Even so, several party strategists said they were concerned about the impact on Republicans running for the state legislature and other offices if he stayed on the ballot.
The Senate election is to replace Republican Peter Fitzgerald, who decided not to seek a second term.
Ryan has been struggling for political survival since Monday, when divorce records were released showing that his ex-wife, actress Jeri Ryan, said he took her to sex clubs and tried to pressure her to perform sex acts while others watched. Ryan has denied the allegations.
In his prepared statement, Ryan sharply attacked the media for its involvement in winning the release of sealed records.
"The media has gotten out of control. The fact that The Chicago Tribune sues for access to sealed custody documents and then takes unto itself the right to public details of a custody dispute — over the objections of two parents who agree that the re-airing of their arguments will hurt their ability to co-parent their child and hurt their child — is truly outrageous," he said.
Although Fitzgerald and the National Republican Senatorial Committee stood by Ryan, he came under immediate pressure from many GOP officials in his home state to relinquish his nomination.
Members of the state's GOP congressional delegation met with House Speaker Dennis Hastert, R-Ill., on Thursday to discuss the issue, and one official said afterward that the speaker concurred that Ryan needed to step aside.
Fitzgerald said Friday that he had encouraged Ryan to stay in the race, calling the response to the scandal "grotesque."
"I told him that it troubled me greatly that so many party leaders who had no trouble stomaching years and years of corruption and insider deals and scandals under George Ryan were now lining up to throw stones at Jack (no relation to George Ryan)," Fitzgerald said.
Ryan, 44, was seen by many as the party's best hope of revitalization after a devastating 2002 election, in which Illinois Republicans lost control of the governor's office and nearly every statewide office, and an ongoing corruption scandal involving former Gov. George Ryan, who has since been indicted.
But those hopes were dashed by the unsealing of his divorce records. Ryan had fought the unsealing, saying it would harm his 9-year-old son. The Chicago Tribune and Chicago TV station WLS sued to have the records released.