Updated at 9:09 p.m.

Rep. Tim Johnson (R-Ill.) will not seek reelection this year, according to senior Republican aides.

Johnson has decided against pressing forward with a general election campaign despite winning his party’s nomination for another term last month. Because the primary is completed, county GOP chairman in the district will be tasked with picking a replacement.

Johnson’s district underwent major changes under a redistricting plan crafted by Illinois Democrats. His new swing district included many new constituents for Johnson, who is known for personally calling each resident as he seeks reelection.

Potential ballot replacements include former state party executive director Rodney Davis, who oversaw Republican gains in Illinois in 2010 and would likely have the backing of Rep. John Shimkus (R-Ill.), who has represented much of the territory in Johnson’s new district.

The other top contender is Johnson’s former chief of staff, Jerry Clarke, who is now with fellow Illinois Rep. Randy Hultgren. Sources say Clarke will run and may have Johnson’s backing for the race to succeed him.

Others mentioned include Johnson’s current chief of staff, Mark Sheldon, along with state Rep. Chapin Rose and 2010 gubernatorial nominee Bill Brady.

The vote among the country chairs will be weighted according to their share of the district, meaning bigger counties will have more say over who the nominee is.

The Democrats also have yet to learn who their nominee will be. Physician David Gill led prosecutor Matt Goetten by 143 votes at last check, but Goetten isn’t conceding.