It was a reprisal of [Hillary's] offensive at Thursday night’s debate, but she escalated her assault further, portraying Mr. Sanders as an impediment to the health care law so associated with Mr. Obama that, she noted, it bears his name.

“He does not support the way I do building on the progress the president has made, and that includes building on the Affordable Care Act,” she said of her opponent, receiving a loud ovation when she mentioned Mr. Obama’s signature domestic accomplishment. It is African-Americans, she added to more applause, who “have made the most advances and gains” thanks to the health law.

Mrs. Clinton’s assault was an indication of how aggressively she intends to run against Mr. Sanders, who routed her by 22 percentage points in the New Hampshire primary this week. Separately, the “super PAC” supporting her, Priorities USA, said it would begin running ads that Mrs. Clinton is the true heir to Mr. Obama’s legacy when it comes to helping blacks.

The electorate in the South Carolina Democratic primary on Feb. 27 is expected to be at least half black, a constituency that reveres Mr. Obama and was crucial in his defeat of Mrs. Clinton in the 2008 Democratic contest here.

Mrs. Clinton still enjoys a substantial advantage over Mr. Sanders among African-Americans in polls, but her campaign is plainly concerned about his making inroads with such voters. Her campaign has conducted focus groups of African-American voters, an aide said, which revealed that many of them associate attacks on the health law as no different from attacks on Mr. Obama himself.

Mr. Sanders voted for the legislation, but has called for going further and enacting universal, government-run health care for all Americans.

The Democrats are turning into the Black Party. Like with the Oscars and 12 Years a Slave, once you go black you are never supposed to go back.