CNN town hall: Sanders slams 'racist' GOP efforts.

Bernie Sanders accused Donald Trump and others of fomenting a "racist effort" to delegitimize President Barack Obama, four days ahead of the South Carolina Democratic primary in which African-American voters will play a crucial role.

In a CNN town hall meeting Tuesday in Columbia, South Carolina, Sanders portrayed the "birther" controversy that Trump pushed during Obama's first term as part of a Republican strategy to thwart his presidency, based on the mantra "obstruct, obstruct, obstruct."

"We have been dealing in the last seven years with an unprecedented level of obstructionism against President Obama," Sanders said.

Sanders has been criticized by rival Hillary Clinton in recent debates and town halls for not being sufficiently supportive of the President, who remains popular among many Democrats, especially African-Americans. Sanders is under pressure to improve his standing among minorities before South Carolina's primary and the string of Southern contests that follow on March 1, which could tilt the momentum of the Democratic White House race toward Clinton.

Clinton is counting on a strong showing in Southern states likely to showcase her dominance among African-American voters, putting the onus on Sanders to try to broaden his support or face falling behind. The Vermont senator clearly appreciates the urgency and is taking an increasingly tough line against the former secretary of state.

He opened the town hall by reiterating his call for Clinton to release transcripts of paid speeches that she made to Wall Street banks after she left the State Department.

"I am happy to release all of my paid speeches to Wall Street -- here it is," Sanders said, with a wave of his hands. "There ain't none."

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