"Trump is clearly not the only person involved in this. There was an effort to try to legitimatise the president," Sanders told a CNN town hall in Columbia South Carolina, ahead of the crucial Democratic primary here over the weekend.
Sanders was responding to a question, if he believed Trump was motivated by racism by raising the birth issue of Obama.
"Look, you can disagree with Obama all you want but to say that the President of the US who won an election fair and square was not a legitimate president, it really undermines what we are as a nation," Sanders said.
Sanders alleged that the birther issue from Trump and others was a racist effort to try to de-legitimatise the president of the US.
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"Can you imagine that? To say he is not really the president. He was not born in the US which is nonsense. You know, it is a funny thing on that issue. My dad, as I mentioned, came from Poland. I am running for president. Guess what? Nobody has asked for my birth certificate. May be it is the colour of my skin, I do not know," Sanders said.
Theories alleged that Obama's published birth certificate was a forgery, that his actual birthplace is not Hawaii but Kenya. Other theories alleged that Obama became a citizen of Indonesia in childhood, thereby losing his US citizenship.
Later, Obama released his original birth certificate and said the controversy surrounding the issue had become a "sideshow".