US Democratic presidential candidate Bernie Sanders has accused the Republican camp of a “racist effort” to delegitimize President Barack Obama.

"We have been dealing in the last seven years with an unprecedented level of obstructionism against President Obama," Sanders said Tuesday at a CNN town hall meeting.

Sanders criticized Republicans for obstructionism in Congress and suggested that those who have questioned whether Obama was born in America are racists.

The Vermont senator took a swipe at Republican presidential front-runner Donald Trump for pushing the “birther” controversy during Obama’s first term in office.

He argued that the movement was part of a GOP strategy to undermine Obama’s presidency. “This is a racist effort to try to delegitimize the president of the United States.”

The comments came just four days before the Democratic primary in South Carolina, which has a large African-American electorate.

Sanders also called for reforms in the criminal justice system and vowed to increase funding for predominately African-American colleges and universities if he is elected president.

The Democratic front-runner, Hillary Clinton, addressed the issue of race in more personal terms.

She said that white people should recognize that “our experiences may not equip us to understand what a lot of our African-American fellow citizens go through every single day.”

"We have serious challenges and I think it is important for people, and particularly for white people, to be honest about those," Clinton said at the town hall. "Otherwise, we are never going to be the nation we should be, we are never going to overcome our legacy."

Clinton narrowly defeated Sanders in the Nevada caucuses on Saturday.

The former secretary of state had lost the New Hampshire primary to Sanders but won the caucuses in Iowa by a razor-thin margin.

Trump won big in the Nevada caucuses on Tuesday night. The outspoken billionaire has stirred controversy on the campaign trail by making xenophobic and disparaging remarks against women, Muslims and immigrants.