A hearing on a lawsuit against Duckworth is set for Thursday.
The ad said Feingold ignored a 2009 memo.
Democrats have targeted Kirk's seat as one they hope will help them retake the Senate.
Reid said Senate Republicans should "stop waffling" on Trump.
"I've got this set of doubts," the purple-state senator says.
The DSCC wants to know if Republicans will meet with the presumptive GOP nominee.
Sen. Mark Kirk said he is the "best-positioned Republican" to avoid being linked to Trump.
The Senate minority leader showed he will be a vocal critic of the presumptive Republican nominee.
Ted Strickland is hitting Sen. Rob Portman over a report that he could be Trump's running mate.
Sen. Richard Burr says he will support Trump as the nominee.