Plummer is running against Democrat William Enyart.
Former regional schools superintendent Brad Harriman abruptly abandoned his campaign Tuesday for the 12th Congressional District seat.
Congressman Jerry Costello says he won’t reconsider his decision to step aside and not seek re-election.
The Democrat endorsed Brad Harriman of O’Fallon, citing his working-class union family and hard work ethic.
The Illinois AFL-CIO has endorsed Democrat Brad Harriman of O’Fallon.