Jerome Corsi arrived at a downtown Nairobi hotel to present his book "The Obama Nation".
Obama is revered in Kenya for his paternal roots here and as a flag bearer for Africa on the international stage.
Corsi’s book, which critics say is meant to hurt Obama’s poll prospects next month, questions whether he could be using drugs and insinuates the Democratic candidate is Muslim rather than Christian.
Local organisers of the book launch said Corsi broke no immigration rules but was considered an embarrassment. A recent poll found that 89 percent of Kenyans want Obama to win the U.S. election, versus three percent for Republican John McCain.
"His papers were found to be in order. His passport was fine, his visa was fine," Corsi’s publicist in Kenya Peter Mbae told Reuters.
"But the government did not want him to launch his book on Kenyan soil. That’s why he has been deported."
Immigration and other officials declined to comment.
An invitation to the book launch said the event would expose Obama’s "deep secret ties" to mafia-like groups in Kenya.
"During his stay, Corsi also plans to meet Sen. Obama’s brother, George Hussein Onyango Obama, who lives on $12 a year in the sprawling Huruma slums and hand him a $1,000 cheque," said the invitation, published in local papers.
Some right-wing U.S. commentators have said the case of George Obama, who is a half-brother, shows Barack Obama’s double standards. But George Obama has told media he is happy with his life and that his case has been exaggerated for political ends.
Corsi also co-authored a book which put 2004 Democratic U.S. presidential candidate John Kerry on the defensive during his unsuccessful election campaign. "Unfit for Command" questioned accounts of an attack on Kerry’s boat during the Vietnam war.
One Obama fan in Nairobi, Jack Okinyi, applauded Corsi’s expulsion. "His actions were highly provocative, that he dared bring such a book here. He was lucky that the authorities got him before Kenyans got their hands on him," he said. Reuters