CHARLESTON, S.C. — As he left Martha Lou’s Kitchen, a soul food institution here on Wednesday, Edward Gadsden expressed irritation about the Republican determination to block President Obama from selecting Justice Antonin Scalia’s replacement on the Supreme Court.

“They’ve been fighting that man since he’s been there,” Mr. Gadsden, who is African-American, said of Mr. Obama, before pointing at his forearm to explain what he said was driving the Republican opposition: “The color of his skin, that’s all, the color of his skin.”

When Senator Mitch McConnell of Kentucky, the majority leader, said after Mr. Scalia’s death on Saturday that the next president, rather than Mr. Obama, should select a successor, the senator’s words struck a familiar and painful chord with many black voters.

After years of watching political opponents question the president’s birthplace and his faith, and hearing a member of Congress shout “You lie!” at him from the House floor, some African-Americans saw the move by Senate Republicans as another attempt to deny the legitimacy of the country’s first black president. And they call it increasingly infuriating after Mr. Obama has sp