Since he represented a historically Republican district, Sullivan was targeted for defeat by the Republicans in the 2004 election. However, he defeated Republican Tom Ernst with 61 percent of the vote. In March 2006 Illinois's 17th congressional district's congressman, Lane Evans, announced he would be retiring at the end of his 12th term shortly after he won the nomination for a thirteenth term. Sullivan, whose district was largely coextensive with the west-central portion of the congressional district, entered the special nominating election finishing second to a longtime aide of Evans, Phil Hare. Sullivan was unopposed for reelection in 2008. In 2012, Sullivan handily defeated Adams County Clerk Randy Frese.