Republican frontrunner Mitt Romney highlighted the gap between himself and average Americans on Tuesday when he maintained he did not make "very much" from speaking fees, even though the $370,000 earned in a single year would be considered a fortune by many middle-class Americans.

His comments came at a press conference while out campaigning in South Carolina ahead of its potentially decisive primary on Saturday. Romney is the favourite to win in South Carolina, enjoying a double-digit poll lead over his nearest rival, former speaker Newt Gingrich.

At the press conference in Florence, he disclosed that he pays a tax rate of only 15% in spite of having an estimated wealth of $200m.

Romney has so far resisted offering details about his financial dealings but, during the presidential debate in Myrtle Beach on Monday night, he finally bowed to pressure and said he would "probably" release his tax returns in April.

He told reporters: "What's the effective rate I've been paying? It's probably closer to the 15% rate than anything."

He said his income came mainly from post-retirement investments. In addition, he said there were speaking engagements: "And then I get speakers' fees from time to time, but not very much." These fees, which he has already disclosed, amounted to $370,000 in the course of one year. The "not very much" could haunt him in a general election.

Romney's remark recalled a previous GOP debate in which he offered Texas governor Rick Perry $10,000 in a bet, an incident that played into the portrayal of him as extremely wealthy and unable to relate to ordinary Americans.

Gingrich, also out campaigning on Tuesday, reiterated the tax issue, asking why Romney had to wait until April and claiming South Carolinian voters had a right to know before the primary. Gingrich has promised to release his tax returns on Thursday.

Gingrich, who feels Romney was weakened in the debate, particularly on the tax issue, told CBS: "If you're a South Carolinian, you say: 'Wait a second, why don't you want me to know about it? Why are you going to wait until after I've voted?' Last night he conceded in principle that he owes it to the public to share it. Strikes me he ought to do it before South Carolina votes."

The criticism of Romney and the potential release of his tax returns in April could help the Democrats gearing up for Barack Obama's re-election campaign. Obama has adopted as one of his main campaign themes that the Republicans are the party of the wealthy, protecting the tax cuts of the rich, rather than helping working- and middle-class Americans.

The multi-millionaire investor Warren Buffet, among others, has argued that the wealthy should pay more in tax and not just the same as, or often less, than ordinary Americans.

Romney is also being criticised by Republicans and Democrats alike over his stewardship of Bain Capital, the private equity firm that invested in companies and, in some cases, laid off thousands of workers. The 15% tax is on income that includes post-retirement investment and profits from Bain Capital.

"My income comes overwhelmingly from some investments made in the past, whether ordinary income or earned annually. I got a little bit of income from my book, but I gave that all away," Romney said.

Romney also discussed the job losses while at Bain Capital.

"Four companies created 120,000 jobs. It's very simple," he said "Four companies created 120,000. Staples, Bright Horizons, Steel Dynamics, and … which one am I missing? Sports Authority."

About 10,000 jobs were lost elsewhere. That amounts to more than 100,000 jobs he created, he said.

The latest poll in South Carolina, by Monmouth University, has Romney on 33%, Gingrich 22%, Rick Santorum 14%, Ron Paul 14%, and Rick Perry 12%.