A top White House adviser on Sunday decried Donald Trump's recent public appearances as a "sideshow," saying the American people will not be swayed by the questions he's raised about President Obama's citizenship.

Polls show that the billionaire reality star's popularity has been on the rise as he's taken up the so-called "birther" cause. He's expected to decide by June whether he's in or out as a presidential candidate, and White House senior adviser David Plouffe suggested the White House is not at all nervous.

"I hope he keeps on rising (in the polls) because ... there's zero chance that Donald Trump would ever be hired by the American people to do this job," he said on ABC's "This Week." "There may be a small part of the country that believes these things, but mainstream Americans think it's a sideshow."

Trump has drawn attention in recent weeks by questioning the authenticity of official documents showing Obama was born in Hawaii -- appealing to a fringe movement that suspects Obama was not born in the United States. Most mainstream politicians have rejected this conspiracy theory as hokum, but Trump has embraced it.

Trump, speaking on CNN's "State of the Union," defended his decision to draw attention to the issue. He said "this is not a conspiracy on my part" and that he'd much rather talk about issues like China, but that the more he looks into the background behind Obama's birth, "the more suspect it is." Though the president has produced a certificate of live birth -- and Hawaii newspapers carried his birth announcement -- Trump said that's not enough. He also lamented the way the so-called "birthers" have been characterized.

"Now, he doesn't have his birth certificate or he's not showing it," Trump said. "So it's a very strange situation. That's all I can tell you. And, you know, it's a very sad thing because the people, the birthers, they got labeled and they got labeled so negatively, and even the word 'birther' is a negative word. ... And the fact is, if he wasn't born in this country, he shouldn't be the president of the United States."

Plouffe, though, described Trump's comments as a "spectacle" and "sideshow behavior" which most Americans reject, and said the public wants U.S. leaders to focus on things like the economy.