Hilary Clinton has replied to Katy Perry after she offered to write a "theme" for her.

The US politician is currently promoting her new book Hard Choices and she recently met up with Perry.

The superstar singer posted a picture of herself on Instagram of herself with the Democrat, saying:

"I told @hillaryclinton that I would write her a 'theme' song if she needs it."

Clinton has now responded, saying: "Well that’s not a Hard Choice. You already did! Keep letting us hear you Roar."

Perry has been a vocal supporter of fellow Democrat Barack Obama in the past.

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Watch Katy Perry in action below: