And it was Bill who introduced Perry.
"Look, I don't want to talk long, because I have never been the warm-up act for Katy Perry before. I am well aware I am the warm-up act," Bill, 69, told the Des Moines crowd, which had gathered before the Iowa Jefferson-Jackson Democratic dinner, according to ABC News.
But Perry's concert – which included "America the Beautiful," "Firework" and "Wide Awake" – was all about Hillary.
Perry, 31, of course, also performed "Roar."
"How many of you are going to be 18 next year, before November? Are you going to vote? Because you know you have the power, right?" Perry told her fans, according to Billboard. "Listen, you have so much power and this is going to be one of the most important elections in over a decade.
"So much change can happen. Let's go Hillary!"
Daniel Acker / Bloomberg / Getty
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In her own remarks, Hillary, 67, thanked Perry for appearing – just one day before her birthday.
"I have to give a special shout-out to somebody really special, somebody whose birthday is tomorrow, someone who reminds us that sometimes you just have to let them hear you roar," Hillary said. "Katy Perry, thank you for being here."
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Perry's final contribution to the campaign was actually a birthday gift, but from her to Hillary, not the other way around: a gold necklace that said 'POTUS'.
Reporting by SANDRA SOBIERAJ-WESTFALL