WASHINGTON (AP) — Talk about an opening act.

Before President Barack Obama spoke to donors at a star-studded fundraiser this month in Los Angeles, 6,000 people at the Nokia Theatre heard from A-list entertainers such as Katy Perry, Stevie Wonder and George Clooney.

Comedian Dennis Miller, meanwhile, has been introducing Mitt Romney at campaign rallies, and musicians Kid Rock and Lee Greenwood have performed for the GOP ticket.

Whatever their political differences, the performers share a common goal: Fire up the crowd before a political rally, energizing supporters and creating all-important buzz as the campaigns look for any advantage in a tight race.

Darrell West of the Brookings Institution, a Washington think tank, says celebrity warm-up acts can validate a candidate but are unlikely to have a major impact.