The sources, who spoke on condition of anonymity, say a formal announcement is expected within hours.
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.
Illinois GOP leaders would select another candidate in the event of a withdrawal. Ryan's replacement would become an instant underdog in a campaign against Democratic State Senator Barack Obama.
Viewers with disabilities can get assistance accessing this station's FCC Public Inspection File by contacting the station with the information listed below. Questions or concerns relating to the accessibility of the FCC's online public file system should be directed to the FCC at 888-225-5322, 888-835-5322 (TTY), or firstname.lastname@example.org.