No Democrat candidates sought their party's nomination, meaning Shimkus will likely be unopposed in the November general election and coast to an 11th term in office.
Shimkus's spokesperson said the congressman would not be making media calls tonight, but issued a statement.
"Thank you to the people of the 15th Congressional District for their support and for allowing me the opportunity to continue to serve them in Washington. I'd also like to thank my opponent for a hard fought campaign.
"To all of our hardworking volunteers and supporters, I cannot thank you enough for the thousands of calls you made and doors you knocked. Your hard work and dedication to spreading our conservative message throughout Central and Southern Illinois was essential to our victory.
"I'm honored and humbled to be able to continue to serve the hardworking families of Central and Southern Illinois."
Vote totals in Shimkus's home turf - Madison County - mirror preliminary results from the district at large, which includes all or part of 33 southern Illinois counties at 56 to 44 percent.
All but one precinct was reporting in Madison County at 10 p.m. Shimkus had 5,209 votes to McCarter's 4,033.