Strategist Mark Penn was the New York senator's chief political adviser until April.
In a 2007 memo, Penn said of Obama: "All of these articles about his boyhood in Indonesia and his life in Hawaii are geared toward showing his background is diverse, multicultural and putting that in a new light. Save it for 2050," CNN reported.
Penn wrote that Obama's past "exposes a very strong weakness for him," noting that the Illinois senator's roots in America are limited.
"I cannot imagine America electing a president during a time of war who is not at his center fundamentally American in his thinking and in his values," Penn wrote.
The contents of the memo were first reported by The Atlantic magazine.