FOX has confirmed that the aide Souder had an affair with is Tracy Jackson, a part-time aide who worked in his northern Indiana district office.
Jackson is still on staff.
She is described as being in her mid-40s and married.
Jackson handled communications for Souder in the district and periodically travelled with him.
According to Legistorm.com, Jackson earned $12,541.70 from Souder's office last year.
She came to work for Souder in December, 2004 as what is described as a "special assistant/communications." She earned $533.33 that year.
Fox has learned that Souder initially discussed retiring at the end of this term, but not standing for re-election. However, after talking with his family over the weekend, decided to resign.
Souder and senior House leaders discussed his exit over the weekend.