Anti-Obama conspiracy theorists are gaining momentum with a new video clamoring for the president’s birth certificate. John Avlon on how the roots of the “Birther” movement lie in the 2008 campaign. Avlon is the author of Wingnuts: How the Lunatic Fringe Is Hijacking America.

Signs of the Birthers’ grassroots conservative conspiracy-theorist momentum have popped at a town-hall meeting hosted by centrist Republican Rep. Mike Castle in Delaware.

A video of the event shows a nascent modern American know-nothing movement rallying around a crazed quest for Barack Obama’s birth certificate. It’s a perfect example of Obama Derangement Syndrome, defined as “pathological hatred for the president, posing as patriotism.”

In the clip, an unidentified woman in red stands up to ask Castle a question, brandishing a tiny American flag and her own birth certificate in a plastic bag. After some preamble, she says: “I want to go back to January 20. Why are you people ignoring his birth certificate?” At this point the crowd goes nuts, in every sense of the word.

“He is not an American citizen,” the woman continues. “He is a citizen of Kenya… I don’t want this flag to change. I want my country back!”

Mike Castle appears taken aback. After hemming and hawing a bit, he says, “If you’re referring to the president, he is a citizen of the United States.”

The video is a glimpse at the ugliest underbelly of American politics: an angry and intolerant Nativism that appeals to people’s paranoid fears of “the other.”

He is shouted down in tones that recall the crowd in Monty Python and the Holy Grail yelling, “Burn the witch!”

Castle continues: “You can boo, but he is a citizen of the United States.”

The congressman’s mild if steady attempt to tamp down this conspiracy theory reminded me of the moment in the 2008 campaign when, during a McCain town hall, an aging supporter accused Obama of being an “Arab.” McCain honorably tried to tamp down that strain of support by simply saying “no ma’am” over and over again.

It’s a sign that Republican centrists are operating at a disadvantage against the party’s wingnut fringe. There is a reluctance to take on the extreme in a full-fledged confrontation, a fight for the future direction of the Republican Party, for fear of provoking grassroots retaliation in a GOP primary. And in this there a tacit admission that the fringe is starting to overlap with the party’s base, pumped up by talk radio and the Internet, bleeding over into cable news and now town halls.

You reap what you sow, and some of this is the result of the “Obama is un-American” drumbeat that began by implication during the 2008 campaign and occasionally erupted in ugly ways, as in a should-be-infamous video taken at an October Sarah Palin rally in Johnstown, Pennsylvania. Its residue was reflected in Young Republican Eric Piker’s Facebook post calling the President “Obama bin Laden” and new YR Chairman Audra Shay’s exposed online comments that the president is “anti-American.”

On right-wing Web sites like Free Republic, there has been not only sympathy for the “Birther in Red” from Mike Castle’s town-hall meeting, but reason for the congressman to be concerned. A comment on the site’s Bungalow Bill’s Conservative Wisdom blog offered this perspective: “Listen to this pansy congressman deal with his constituents. This congressman better be worried about his job in 2010 if this is the best answer he has, which is similar to an answer Roy Blunt gave me. Why are Republicans protecting Obama in this matter?”

Of course, it’s not the first time that what Richard Hofstadter called “the paranoid style in American politics” has reared its head. The original Know-Nothing Party of the 1850s was anti-immigrant and anti-Catholic while also waving the American flag, ultimately dubbing itself “the American Party.” And Obama Derangement Syndrome was preceded by liberals’ Bush Derangement Syndrome, a hysterical tendency to equate our then-president with Adolf Hitler.

In the Birther movement, Obama’s alleged birth in Kenya makes him the ultimate illegal alien. And when the Birther in Red speaks emotionally about her father’s service in World War II as a legacy she wants to honor and defend, she tellingly forgets that Obama’s grandfather and grand-uncles served in World War II, as well.

But the town-hall video’s most revealing moment comes when, overflowing with emotion, the Birther in Red nearly screams, “I don’t want this flag to change. I want my country back!”

She wants her country back from a man whom she sees as a usurper to the presidency in part because of his politics and his alleged subversion of the Constitution, but also in part because of his race. Somewhere slithering around is the idea that they must get this black man out of their historically white House.

The video is a glimpse at the ugliest underbelly of American politics: an angry and intolerant Nativism that appeals to people’s paranoid fears of “the other.” Its craziness is starting to seep in at the margins and affect our civil discourse. And its appearance so soon after the election does not bode well for the uniting of our country after the bitterly divided red versus blue decade, and the hope that something new might appear in the Obama era.

John P. Avlon is the author of Independent Nation: How Centrists Can Change American Politics. He writes a weekly column for The Daily Beast. Previously, he served as chief speechwriter for New York City Mayor Rudy Giuliani and was a columnist and associate editor for The New York Sun.