COLUMBIA, S.C. — One day after President Trump claimed he had done more for black voters than the Obama administration had, several Democratic presidential candidates spoke at the same historically black college on Saturday and issued a blistering rebuke of both the president and the event’s organizers — whom they accused of giving Mr. Trump a n undeserved platform.

Senator Kamala Harris of California led the charge, and was set to boycott the event altogether until the group that had invited Mr. Trump was removed as a sponsor. Other candidates, including Joseph R. Biden Jr., Senator Bernie Sanders of Vermont, and Mayor Pete Buttigieg of South Bend, Ind., said the president had a history of racist demagogy that outweighed any recent efforts at criminal justice reform, including the First Step Act , which he signed into law last year.

The First Step Act was a bipartisan bill that helped thousands of federal inmates secure early release under new sentencing guidelines, but Democrats were united in framing it as an insufficient measure. They called for more structural reforms to the criminal justice system, and took aim at Mr. Trump himself, who has made exploitation of racial grievance a trademark of his political brand.

“I find it hypocritical of him to tout whatever advances have been made in the First Step Act given his history,” Ms. Harris said. “The hypocrisy is deafening.” Mr. Biden spoke last, and joked to an audience that had been there for most of the day that they probably felt “incarcerated” themselves. He arrived and left to applause.