Democratic Arizona Rep. Ann Kirkpatrick says she will challenge veteran Republican John McCain for his Senate seat next year.
Kirkpatrick is serving her third House term in a district she won narrowly in 2014.
The 65-year-old former prosecutor released a statement saying she's worked all her life "to put Arizona first."
McCain will be running for his sixth six-year Senate term.
The 78-year-old was his party's unsuccessful presidential nominee in 2008. A prisoner of war during the Vietnam War, he has long been one of his party's leaders on military and foreign policy issues and is chairman of the Senate Armed Services Committee.