By Michael Muskal

Los Angeles Times (MCT)

LOS ANGELES — They say that you can learn a lot about a person from his enemies, but what does this trifecta of stars — Robert De Niro, Jerry Seinfeld and Charlie Sheen — say about politics and Donald Trump?

The three entertainers have all taken shots recently at Trump, the self-promoting businessman who is considering a run for president. While no one really knows if he is serious about running, the publicity that he has generated, particularly for once-dormant questions about President Barack Obama's citizenship, has become serious enough in a weak Republican field to push Trump into the top tier of GOP hopefuls.

Republican politicians — including Arizona Sen. John McCain, House Minority Leader Eric Cantor of Virginia and New York Mayor Michael Bloomberg (now an independent) — have thrown cold water on Trump's campaign on the birther issue. But no one has been able to derail Trump.

Comedian Seinfeld has feuded with Trump over politics and other issues and even Charlie Sheen, the latest self-destructive bad boy of television, has taken shots at Trump for allegedly trying to gift the actor with fake jewelry. Over the weekend, De Niro, an Academy Award-winning actor, slammed Trump in an NBC News interview.

"I won't mention names, but certain people in the news the last couple weeks, just, what are they doing?" said De Niro. "It's crazy. They're making statements about people that they don't even back up. Go get the facts before you start saying things about people.

"It's like a big hustle. It's like being a car salesman," said De Niro, who later said he was referring to Trump. "Don't go out there and say things unless you can back them up. How dare you? That's awful to do. To just go out and speak and say these terrible things? Unless you just wanna get over and get the job. It's crazy."

Never shy, Trump fired back on Monday in an interview on "Fox & Friends."

"I like (De Niro's) acting, but in terms of when I watch him doing interviews and various other things, we're not dealing with Albert Einstein," Trump said. "He can say what he wants, but the fact is that this guy (Obama) has not revealed his birth certificate. A lot of people agree with me."

Trump has successfully rode the birth certificate issue to a top spot in some polls and in all polls to at least a contending position among the more than a dozen possibilities mentioned in the GOP sweepstakes. His name recognition has helped fuel his rise along with what seems to be an almost daily appearance on some media outlet.

But Trump has insisted that he does have political positions, including getting tough with China on economic issues and taking on OPEC. He has said he is more militaristic than Obama and would seize oil wells in Iraq to prevent them from falling to Iran.