For President Biden, who is engaged in the fight of his political life, everything is politics these days, even going to church.

On Sunday morning, Mr. Biden spoke at a Black church service in Philadelphia, seeking to reassure the group of voters who helped him win the White House in 2020 that he is still capable of beating former President Donald J. Trump.

“The joy cometh in the morning,” Mr. Biden told several hundred people at Mount Airy Church of God in Christ, a political fixture in the city often visited by Democrats. “You’ve never given up in my life. And as your president, I’ve tried to walk my faith.”

Before Mr. Biden spoke, Rachel Hooks, a member of the church, offered a prayer that did not shy away from the doubts swirling around his candidacy.

“Touch his mind, O God, his body; rejuvenate him and his spirit, O God — bless him and give him direction,” Ms. Hooks said, echoing a similar plea for mental and physical strength she also offered for members of law enforcement.

During his speech, Mr. Biden spoke from notes, rather than from the teleprompter that usually guides his public addresses, and his remarks at points seemed jumbled and could be difficult to hear.