In tight race, Illinois Sen. Mark Kirk steps away from GOP

Kirk's approach varies from Republicans in competitive races in states such as Ohio and Pennsylvania, who may be uneasy about the party's presumptive presidential nominee, Donald Trump, but are campaigning on their conservative records.

Kirk, who represented Chicago's North Shore in the U.S. House before winning Obama's former Senate seat in 2010, insists he has always had an independent streak; he has supported abortion rights and gun-control legislation since early in his five-term House tenure.

Duckworth's campaign also says Kirk talks a lot about his more moderate positions when he's in the Chicago area, but not when he heads to more Republican parts of Illinois, and that she has been consistent in her beliefs.

Asked this week about Indiana Gov. Mike Pence, Trump's pick for vice president and a social conservative who last year signed legislation critics said could have let businesses in the state refuse to serve gays, Kirk said "I love Mike."

The votes followed an attack at a gay nightclub in Orlando, Florida, that left 49 people dead and was the worst mass shooting in modern American history.

Kirk, meanwhile, has seemed to go to some lengths to avoid being seen rubbing shoulders with top Republicans, being a last-minute no-show for party festivities at last summer's Illinois State Fair and this year's state party convention.