Vice President Kamala Harris warned on Monday that American freedom was “under profound threat” in a speech honoring the Rev. Dr. Martin Luther King Jr. in South Carolina, amplifying a message that the Biden administration has made a rallying cry of its re-election bid.

Ms. Harris used her keynote address at a South Carolina N.A.A.C.P. event to highlight curbs placed on civil rights by Republican lawmakers and the Supreme Court in recent years. She urged the crowd of more than 100 to continue to fight for the constitutional promises Dr. King spent his life holding America accountable for.

“If he were here, I think Dr. King would be the first to say that yes, we have come far, and though we have come far, in this moment it is up to us to continue that fight to cash that promissory note,” she said, making a reference to his “I Have a Dream” speech.

Ms. Harris criticized “extremists” who have passed laws rolling back voting rights and reproductive rights, orchestrated book bans and denied parts of the nation’s Black history.