Former vice president Joseph R. Biden Jr. was once a clear front-runner in the race for the Democratic nomination. Now, he is fighting back from a string of losses and staking his candidacy on his ability to win tomorrow’s South Carolina primary, the first in a state with a large black population. But will he win, and if the margin isn’t as decisive as he hopes, can he stay in the race? Guest: Astead W. Herndon, who covers national politics for The New York Times traveled to South Carolina with Clare Toeniskoetter and Annie Brown, producers on “The Daily.” For more information on today’s episode, visit nytimes.com/thedaily.  Background reading: A new poll showed Mr. Biden with a wide lead in South Carolina, with Senator Bernie Sanders and hedge fund billionaire Tom Steyer trailing behind.Mr. Biden lashed out after reports that Mr, Sanders considered mounting a primary challenge to President Barack Obama in 2012, saying it was “one of the reasons I resent Bernie.”Churches have long played the primary role in mobilizing black support in South Carolina. So how are candidates faring among congregations?