Washington: Ahead of a major nomination contest in South Carolina, Democratic presidential candidate Bernie Sanders accused Republican presidential front-runner Donald Trump of fomenting a "racist effort" to delegitimise President Barack Obama.
Appearing at a CNN town hall on Tuesday 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," the self-styled Democratic Socialist said.
Democratic presidential frontrunner Hillary Clinton has criticised Sanders in recent debates and town halls for not being sufficiently supportive of Obama, America's first African-American president.
African-American voters are expected to play a crucial role in South Carolina and the string of Southern contests that follow on March 1, which could tilt the momentum of the Democratic White House race toward Clinton.
Addressing the race issue, Sanders called for reforms in the criminal justice system and promised to hike funding for historically black colleges and universities if he is elected president.
Clinton responded in more personal terms, saying that white people should be honest and recognize "that our experiences may not equip us to understand what a lot of our African-American fellow citizens go through every single day".
Sanders, who is posing a tough challenge to Clinton, also reiterated his call to the former secretary of state 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."
Clinton, who appeared on stage after Sanders, sidestepped questions about the senator's call for her to release her speeches.
"If everybody does it, and that includes the Republicans -- because we know they have made a lot of speeches," Clinton said.
She said the real issue was about who had the best plan to crack down on Wall Street.
Meanwhile, a federal judge paved the way for possible future subpoenas by the State Department against Clinton and her long time Indian-American aide Huma Abedin to obtain personal emails.
Clinton told her supporters that they had nothing to fear from the controversy, which she said was part of a long succession of episodes that turned out to be nothing.
"I am well aware of the drip, drip, drip. I have been in the public arena for 25 years," she said. "The facts are that every single time somebody has hurled these charges against me, which they have done, it has proved to be nothing."