Nowhere was Mr. Steyer’s money more evident than in South Carolina. As his rivals fought amid the crowded Democratic field to gain a foothold in Iowa and New Hampshire, Mr. Steyer’s campaign saw an opportunity to saturate South Carolina voters with television ads and direct mail while also building out a large campaign operation and making dozens of visits to the state.

But Mr. Steyer’s campaign went beyond even those efforts. In a state where black voters comprised about 60 percent of the Democratic electorate, Mr. Steyer’s campaign hired local African-American staff members, used black-owned businesses to cater events and provide other services and committed to spending about $150,000 on advertising in the black-owned media.

The heavy spending strategy — which presaged a similar approach taken by fellow billionaire Michael R. Bloomberg — appeared to be at least partially responsible for the boost in the polls that Mr. Steyer enjoyed in South Carolina. In January and February, Mr. Steyer polled in the double digits several times in surveys of South Carolina voters despite having never performed that well in dozens of other national and early voting state polls, save for once or twice in Nevada.

But exactly how Mr. Steyer wielded his financial resources also came under scrutiny and raised questions about whether he was seeking to gain influence though his spending. For instance, Mr. Steyer’s campaign made commercial rent payments to a company owned by Jennifer Clyburn Reed, a daughter of James E. Clyburn, the longtime Democratic congressman from South Carolina whose endorsement was highly coveted and eventually went to former Vice President Joseph R. Biden Jr.

Mr. Steyer’s campaign also put influential black lawmakers on his campaign payroll as advisers — some of whom endorsed him — and faced allegations of vote-buying, all of which raised concerns among some South Carolina Democrats. Earlier in the cycle, Mr. Steyer’s campaign was forced to apologize for what it said was “miscommunication” following a report that a top aide had privately offered campaign contributions to local politicians in exchange for endorsements.

Nonetheless, Mr. Steyer’s relative strength in South Carolina earned him one final chance to make his case in front of a national television audience at the debate in late February.