Most political town hall meetings are pretty dull affairs. Not this one.

The YouTube clip of an irate woman waving her birth certificate in a plastic bag and telling Delaware congressman Mike Castle that Barack Obama was born in Kenya, not the United States, has become an online sensation. Castle, a mild-mannered moderate, was booed when he insisted that our president is in fact a citizen. It's been on the Drudge Report, Rush Limbaugh and just about every political website out there. The session was so embarrassing that a Castle staffer felt obliged to tell a local columnist that the hecklers were from out of state.

The birther controversy has a tight grip on rightwing opponents who argue that Obama is not the lawful president. FactCheck.org took a thorough look at the issue – and the birth certificate itself – and concluded: "It meets all of the requirements from the state department for proving US citizenship."

The website went further: "We suggest that those who choose to go down that path should first equip themselves with a high-quality tinfoil hat." But the birthers seem immune to the lack of evidence. As Michael Tomasky points out: "He can't prove he's not a space alien either."

The nutty theories didn't end with the birth certificate. Members of the crowd told Castle that global warming is a hoax and just a theory like evolution. You can't tax carbon dioxide, he was told, because "trees need CO2 to make oxygen!"

The session got even nuttier. Castle listened as one audience member insisted that the swine flu virus was engineered in "a small bioweapons plant outside of Fort Dix", and that a big vaccine company was "caught sending Aids-infected vaccines to Africa". This speaker continued: "You think I'm going to trust you to put a needle full of dead baby juice and monkey kidneys? Cause that's what this stuff is grown on, dead babies!" The episode neatly encapsulates the dilemma of the Republican party. Castle could be the party's best shot at taking a Senate seat next year – if he chooses to run. But he has drawn the ire of the extreme rightwing for his vote for the Waxman-Markey cap-and-trade bill, derided by rightwingers as "cap and tax" or "cap and traitor".

Castle exemplifies the kind of centrist who kept the Republican party relevant in this increasingly blue state. He is the last living relic of Delaware's heritage as a one-time swing state, having been elected governor twice and congressman eight times. Simply by considering a run, he is giving attorney general Beau Biden, Joe Biden's son, second thoughts about going for the Senate seat being warmed by Biden loyalist Ted Kaufman.

But Castle must be wondering whether it would be worth enduring this kind of nuttiness to crown his long career with a campaign for a Senate seat.