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.
Keith Laing 03/17/15 05:29 PM EDT
Illinois state Sen. Darin LaHood (R) has been mentioned as a potential candidate.
Rep. Robert Dold is ninth Republican to back bill.