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.
The embattled Illinois Republican will step down amid allegation of improper spending.
Dems could face primary before fight against embattled Rep. Robert Dold (R).