Joseph P. Riley Jr., the former 10-term mayor of Charleston who supports Mr. Biden, remembered the candidate’s visit in the aftermath of the 2015 massacre at Emanuel African Methodist Episcopal Church. Mr. Biden was in mourning, too, weeks after his son, Beau, had died of brain cancer. Following a church service, Mr. Riley said, he led Mr. Biden to the office of the pastor who had been killed, the Rev. Clementa C. Pinckney, to show him something.

“He only had one photograph on the wall. It was a photograph of Rev. Pinckney and Joe Biden,” Mr. Riley recalled. “I didn’t say anything, didn’t have to say anything. He understood why I brought him down there.”

Mr. Riley suggested that the inauspicious start to the primary season had only further endeared Mr. Biden to the state, infusing his bid with an urgency that perhaps only losing can deliver. “We all do better as an underdog,” he said. “I was a little football player in high school. You play better when you know the other team might be a little stronger.”

For a politician who has long been sensitive to the reception he receives in a room, his return to the South has reinforced a tale-of-two-Bidens dynamic on conspicuous display for months.

In Iowa and New Hampshire, in front of reserved and often skeptical crowds, Mr. Biden could be stiff and subdued, speaking so quietly he was sometimes difficult to hear, and often pursuing so many tangents that some voters said he was difficult to follow. (Less than two weeks before the Iowa caucuses, for example, he went on an extended riff about the Luddites.) Iowa, the state where his first two presidential campaigns were torpedoed, was also the site of a string of gaffes over the summer that revived an old narrative about Mr. Biden’s indiscipline.

The misstatements and digressions hardly disappeared when Mr. Biden set foot in South Carolina. But here, in front of crowds that are often more diverse and almost always more affectionate than those he encountered in the earliest states, Mr. Biden has also often appeared more lively, more comfortable and, sometimes, sharper.

At a CNN forum on Wednesday night, he contrasted himself pointedly and consistently with Mr. Sanders — never a given for Mr. Biden, a decorous senator at heart who sometimes struggles to press a sustained case against his opponents. On Thursday, in Conway, S.C., he let fly a mildly Trumpian boast about crowd size, telling his audience that the fire code kept “a couple hundred people” outside (the number appeared to be a bit smaller) and noting that he stopped to talk with those “who couldn’t get in.”