“I HAVE a very large Twitter,” Donald Trump is saying.

He is talking to Greta Van Susteren of Fox News.

Van Susteren is unruffled. She does not ask Trump what on Earth he is babbling about. Why bother? Trump has only one subject upon which he speaks with authority: himself.

This day, Tuesday, is supposed to be about Mitt Romney and how he will clinch the Republican nomination for President after finally defeating a field largely populated by non-entities, also-rans and would-be has-beens.

ut the Romney campaign has done its deal with the devil. In return for Donald Trump's support and a Trump fundraiser, it must accept Donald Trump and his ability to dominate the political news cycle with whatever he wants to spew.

Trump has embraced birtherism, the belief that Barack Obama was not born in the United States. The subject is not new. Way back in August 2008, three months before Obama won the presidency, the Honolulu Star-Advertiser was reporting that Hawaii's Department of Health was receiving weekly requests for a copy of Obama's birth certificate.

But Trump's celebrity status — the only status he possesses — has breathed new life into the movement. “I think it is more likely that he wasn't born here,” Trump tells Van Susteren.

The racism that motivates many birthers — the belief that a black man cannot possibly be a legitimate President — does not impede Trump's reviving it as an issue for the fall election.

“I think this is a great issue for Mitt Romney,” Trump says blithely to Van Susteren.

As for Mitt Romney, himself, he says he believes Obama was born in Hawaii. But Romney will not criticize Trump, let alone denounce him.

“You know, I don't agree with all the people who support me,” Romney tells reporters. “My guess is they don't agree with everything I believe in. But I need to get 50.1 percent or more, and I'm appreciative to have the help of a lot of good people.”

Romney's desire to do anything to become President does not make him unique. But the naked political lust behind that statement should win him some kind of award for brazenness.

The “need to get 50.1 percent or more” — let's assume Romney is talking about the Electoral College and not the popular vote — can justify all behavior, including standing on a stage with birtherism's most high-profile voice.

Trump is unstoppable this day. He is talking to Fox and CNN and CNBC. But he is doing this only by phone. He is not prepared to appear on screen. Perhaps he is getting ready for his big appearance at his Las Vegas fundraiser for Romney. Perhaps Trump is getting his hair rust-proofed.

Newt Gingrich will also be at the event. But Gingrich and Trump are two different people. Gingrich occasionally thinks before he speaks.

Asked about Trump and birtherism, Gingrich says carefully: “I think Donald Trump said what he said because he thinks it's the right thing for him to say.”

“Far be it for me to suggest to The Donald what he should do,” Gingrich also says. “Donald's done what he could to make [things] more exciting. He has a knack for that. He's mildly amazing.”

Trump would disagree only with the “mildly.” Bigness is an obsession with Trump. When he was considering running for President, he said, “I'm a much bigger businessman. … I mean, my net worth is many, many, many times Mitt Romney's.”

As to Trump's claim to having “a very large Twitter,” largeness is relative. As I write this, Trump has 1.2 million Twitter followers, Obama has 16 million, and Lady Gaga has 24.9 million.

But Trump is America's leading vulgarian and a relentless pitchman.

“I've been known as being a very smart guy for a long time,” Trump tells CNBC Tuesday.

Then he goes on CNN with Wolf Blitzer and begins talking about birth certificates and Obama's mother not being in the hospital in Hawaii when Obama was born, and Blitzer — praise be unto him — decides to interrupt the blather.

“Donald, Donald,” Blitzer says, “You're beginning to sound a little ridiculous.”

Trump is astounded. And he strikes back with all the finesse of a pudgy schoolyard bully. “No, I think you are, Wolf!” he says. “Let me tell you something, I think you sound ridiculous. … If you would report it accurately, you would get better ratings, which are pretty small.”

Wolf is small. And Trump has a large Twitter.

Republicans are hard-pressed to salvage the day for Romney. “Don't make this Mitt Romney's fault,” David Frum, a former speechwriter for George W. Bush, says to Blitzer. “It is Donald Trump's fault.”

But is it really?

Trump tells Jon Ward of The Huffington Post that the Romney people have not asked Trump to shut up about birtherism. “They haven't done that,” Trump says.

And why should they? Maybe birtherism will get them to 50.1 percent.

Roger Simon is Politico's chief political columnist.