Republican U.S. Sen. Mark Kirk says Donald Trump’s refusal to say he’ll accept the outcome of election is another sign the GOP presidential candidate is “unprepared and unfit to be commander in chief.”

Kirk said Thursday that he will accept the results of his own race for re-election against Democrat Tammy Duckworth.

Kirk says the U.S. has peacefully transitioned power from one president to the next “since the end of George Washington’s presidency.”

He says, “only someone as delusional as Donald Trump would believe he is the exception to this rule.”

Kirk was one of the first Republicans to un-endorse Trump and has been a vocal critic.