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LAS VEGAS - Pop star Katy Perry and 13,000 Nevadans rocked together for President Obama under the stars at a midnight ET rally here to close out the first day of Obama's whirlwind two-day campaign blitz.

Perry, wearing a skin-tight, white dress designed as a ballot, urged the crowd to vote for Obama - and vote early. She sang a full set of her hits, complete with back-up dancers, full concert lighting and a digital backdrop that showcased pictures of Obama and his slogan "Forward."

Obama hailed Perry has "just a wonderful young lady" and said at the top of his remarks that he had the honor of meeting her grandmother backstage - and that she got some of her lipstick on him.

"Fortunately somebody wiped it off before Michelle saw it," Obama said with a big grin.

Perry's grandmother, who is 91 years old, watched Obama's remarks from a chair in the middle of the concert stage as the singer stood nearby. ABC News asked her about the kiss backstage after the event and she said it was "wonderful."

The Vegas stop was the fourth of the day for Obama, who is on a 48-hour, 4500-mile campaign tour through eight states.

"We are pulling an all-nighter. No sleep. And if you're not gonna sleep, you might as well be in Vegas," Obama said.

After the event on his way to the airport, the president made an surprise visit to the Bellagio Hotel, where he greeted hundreds of hotel and casino workers crowded inside.

"Everyone here knows what it means to work hard," he told them. "You're what makes Las Vegas run. You're what makes the strip run."

"I need you guys to go out and vote now. Don't wait to vote," Obama added. "You gotta go cash in your chips now."

The president flies overnight - sleeping on Air Force One - to Tampa, Fla., where he holds an event Thursday morning.