COMMERCE, Mich. -- Republican presidential hopeful Mitt Romney has raised the discredited rumour that President Barack Obama was not born in the United States and is thus ineligible to be president.

Romney joked at a Michigan rally on Friday that he had been born in a nearby hospital and that "no one's ever asked to see my birth certificate."

The Obama campaign swiftly responded that Romney was directly enlisting himself in the discredited "birther movement," and that "should give pause to any rational voter," said spokesman Ben LaBolt.

Obama has been dogged by questions about his birthplace and his Kenyan father. He released a long-form copy of his birth certificate last year, showing he was born in Hawaii in 1961.