Rep. Gary Peters is ahead of Michigan Secretary of State Terri Lynn Land.
Targeted Senate races include Alaska, Iowa, Montana, Arkansas, Louisiana and North Carolina.
They're using ruling against the contraception mandate to rally their base ahead of November.
The League of Conservation voters is backing Collins for reelection.
At rally for challenger, Warren said McConnell is more interested in protecting billionaires.
But the senators are also potential liabilities for vulnerable Dems.
Mississippi attorney Mark Mayfield played a role in the challenge of Sen. Thad Cochran.
Sen. Kay Hagan (D-N.C.) has a narrow lead over North Carolina Speaker Thom Tillis (R), according to a new poll conducted by the Civitas Institute, a conservative North Carolina think tank.
Three Republicans are vying to take on Sen. Mark Begich (D-Alaska).
State Rep. Joe Carr is attacking the senator on immigration in his first TV ad.