Republicans are racing to link Democratic candidates to Clinton.
Sen. Richard Burr (R-N.C.) leads Deborah Ross by 1 point.
"Sincere apologies to an American hero, Tammy Duckworth," Kirk wrote.
The former Red Sox pitcher has discussed running against the liberal senator.
The Senate battle is in a virtual tie, a new poll finds.
"It's really not that hard to grasp," Clinton tweeted to Mark Kirk.
Maggie Hassan and Kelly Ayotte clashed again on stage.
"I forgot that your parents came all the way from Thailand to serve George Washington."
"I don't think Senator Warren speaks for New Hampshire," Sen. Kelly Ayotte said.
The GOP senator is sparking quick backlash from Democrats.