With the hit show "American Idol" averaging about 18 million viewers, maybe presidential candidates are catching on to the singing trend -- serenading their way to voters' hearts.

GOP presidential candidate Mitt Romney exercised his crooning skills at an end-of-the-campaign rally in Charleston, S.C., Friday by singing "Happy Birthday" to South Carolina Gov. Nikki Haley. "She turns 40 years old," Romney said.

Romney also described Haley as a "strong unwavering conservative" and followed his performance with some birthday cake.

"All I want is President Romney for my birthday," said Haley, who has endorsed Romney.

Of course, Romney is not the only in politician wooing voters through song.

President Obama sang the first few words of soul-singer Al Green's song, "Let's Stay Together" Thursday night at the Apollo Theater in New York for a fundraiser, resulting in an effusive reaction from the crowd. "I'm so in love with you ..." Obama sang.

After all the applause, Obama added, "those guys didn't think I would do it" -- not specifying who "those guys" are. 

Even Herman Cain, former CEO of Godfather's Pizza and ex-presidential candidate, recited a few lines from the "Pokemon" movie and broke into song, singing "Believe in Yourself" at Stephen Colbert's "Rock You Like a Herman Cain" rally Friday. 

This comes after a video appeared in October 2011 of Cain singing John Lennon's "Imagine" with pizza-themed lyrics at a Godfather's event. Some of the lyrics included "Imagine there's no pizza," "Eating only tacos, or Kentucky Fried" and "You're lucky you have pizza."