Mitt Romney's opponents have persisted in attacking him over how he made his vast fortune, but the issue so far has found little response in the electorate.
The latest assault came from Newt Gingrich, the former speaker of the House of Representatives, who said the former Massachusetts governor had engaged in exploitive capitalism as a businessman. The effectiveness of that attack will be tested in the South Carolina primary on Saturday. Mr Romney has already won, if narrowly, the Iowa caucuses and the New Hampshire primary.
Conventional wisdom holds he will have the nomination locked up if he continues that winning streak in South Carolina, the first Southern state to vote. That would make him the party's standard-bearer against President Barack Obama in the November election.
Mr Romney says his business experience makes him better prepared than Mr Obama to rebuild the wobbly US economy. The slow recovery from recession has left him vulnerable as he runs for a second White House term.
After resisting calls to open his tax records for public scrutiny - a tradition for presidential candidates in recent decades - Mr Romney this week said he would consider in April making his returns public. April is the month when Americans must file income taxes with the federal government. Many candidates in the past have baulked but always ended up releasing their returns.
He said the returns would show he paid about 15% of his income to the federal government - about 20% less than the top rate assessed on the highest earners in the United States.
Mr Romney is sensitive about his money because increasing numbers of Americans are angry about the extraordinary growth in the percentage of the country's wealth that is in the hands of the few.
Mr Romney's fortune is thought to largely be a result of his work as head of Bain Capital, a firm that specialized in buying up other companies. While some prospered, others did poorly, and Mr Romney's opponents charge him with sucking capital out of the failing enterprises, laying off workers and allowing the firms to collapse into bankruptcy.