His spending has not been without controversy. Allegations of vote-buying involving Mr. Steyer cropped up on social media last week, after the pastor of Brookland Baptist Church, one of the largest black churches in the state, announced during Sunday services that the church planned to apply for a grant from a charitable foundation established by Mr. Steyer and Ms. Taylor.

The pastor, the Rev. Charles B. Jackson Sr., said the money was needed to renovate a church-owned center that provides after-school and athletic programs for children. That same day, Ms. Taylor was scheduled to hold a campaign event at the church. The campaign has said that the couple have stepped away from their foundation during the race. Mr. Jackson could not be reached for comment.

Mr. Steyer and his wife have also come to the aid of Optus Bank, a majority-black-owned bank in Columbia. Optus has received a $1 million deposit from Beneficial State Bank, an institution based in Oakland, Calif., and founded by Mr. Steyer and Ms. Taylor that makes loans in underserved communities and to nonprofit organizations. A son-in-law of Mr. Clyburn, Cecil Hannibal, serves on the bank’s board of directors, according to Dominik Mjartan, Optus’s president.

In an interview, Mr. Mjartan said the deposit came in 2017 — well before Mr. Steyer became a presidential candidate — after he requested assistance from Beneficial State to help his bank stay afloat.

“The bank was in financial distress,” Mr. Mjartan said, adding that he knew Ms. Taylor through her work in community banking. “This is an apolitical effort to make sure that capitalism works for everybody regardless of their ZIP code,” he said.

Ms. Reed said that, like the bank, she and her husband had been working to improve the Columbia community; in August they decided, through their company, to buy two blighted buildings on Gervais Street that were in foreclosure.

Shortly afterward, Ms. Reed said she learned that the Steyer campaign was looking for office space. “Their first contract fell through,’’ she said. “They needed some place stat.”