Former Rep. Brad Scheider will seek to regain his old House seat in a rubber match.
Pete Gallego will again take on Rep. Will Hurd (R), who beat him in 2014.
Raja Krishnamoorthi, who lost to Duckworth in the Democratic primary in 2012, will run for the House again in 2016.
Congress typically is only in session four days a week.
The GOP dissenters included hard-line conservatives and centrists.
The former vice president headlined the NRCC's annual fundraising dinner.
Rep. Dan Benishek (R-Mich.) says he wants to continue to work on veterans' issues.
Democrats narrowly topped the GOP in fundraising last month, $5.2 million to $5 million.
Rep. Steven Horsford (D) was upset in 2014 by Rep. Crescent Hardy (R).
The Illinois state senator is the son of former Transportation Secretary Ray LaHood.