Duckworth was widely favored to defeat Kirk in the heavily Democratic state.
National Republicans largely did not advertise on Kirk's behalf.
Democratic Illinois Rep. Tammy Duckworth has ousted incumbant GOP Sen. Mark Kirk, according to a CNN projection.
It was the Republican Senate seat seen as the likeliest to flip in 2016, with Duckworth widely favored to defeat Kirk in the heavily Democratic state.
Kirk was unable to sufficiently outrun GOP presidential nominee Donald Trump, though he eventually renounced him and said he would not vote for his party’s standard bearer. Kirk later said he wrote in former Gen. David Petraeus for president.
National Republicans largely did not advertise on Kirk’s behalf, viewing the Illinois seat as hard to defend. Kirk also hurt his chances in a recent debate, when he disparaged Duckworth’s mixed-race background and family history.