Republican presidential primary: Sex, taxes and indignation as Newt Gingrich and Mitt Romney vie for Republican ticket

Up until a few days ago, the former Massachusetts governor seemed to be heading towards a coronation in the southern state, with poll numbers predicting a comfortable win.

However, in a few dramatic hours on Thursday, he had his win in Iowa overturned before watching Texas governor Rick Perry drop out, then endorse his main rival, Newt Gingrich.

Those developments and renewed questions over Mr Romney’s financial affairs has seen his lead narrowed.

One poll suggested he has been replaced at the top in the run-up today’s vote.

A strong showing by the former House speaker Mr Gingrich could result in further withdrawals, with former Pennsylvanian senator Rick Santorum perhaps doing as Mr Perry did and backing Mr Gingrich.

Such a move would result in a further surge for him, leading to a likely two-horse race for the White House nod.

But in a further twist, Mr Gingrich is having to confront difficult questions of a personal nature.

The thrice-married politician had to face down an attack on his moral authority after his second wife gave a revealing interview on Thursday in which she claimed he wanted to have an “open” marriage.

Such marital arrangements are not in keeping with Mr Gingrich’s espoused family values, nor do they sit well with the evangelical Christians who go to the polls today.

Mr Gingrich attempted to put an end to questions over his personal life during a fiery performance at a candidate’s debate on Thursday night.

As the encounter opened on the topic, the 68-year-old turned his guns on reporters.

Hitting out at the “despicable, vicious and negative nature” of parts of the media, he focused his eyes on debate mediator, CNN’s John King.

“It is as close as despicable as anything I can imagine. I am frankly astounded that CNN would take trash like that and use it to open up a presidential debate,” he said.

The fight for a Republican White House nod looks to be shaping up to be a two-horse race between Mr Gingrich and Mr Romney.

The Massachusetts Republican has spent the last couple of days fending off questions about his wealth, taxes and record as a businessman.

In Thursday’s debate, Mr Romney was attacked over both his healthcare policy and his refusal to come clean over his finances.

Mr Gingrich released his income tax return shortly before the debate, and he urged the frontrunner to do likewise.

“If there is anything that’s in there that is going to help us lose the election, we should know before the election. If there is not, why not release it?” he asked.

Mr Romney indicated earlier this week that he would release his tax forms in April.

The former businessman, who sits on a personal wealth in excess of $200 million (£129m), did, however, reveal that he pays a tax rate in the region of 15 per cent – lower than that of the average American.

A poll released after the debate suggested that he had slipped below Mr Gingrich in South Carolina.

The Public Policy Polling survey found that 35 per cent of Republican voters were backing Mr Gingrich, compared to 29 per cent behind Mr Romney.

Mr Romney could still win tonight, but in any account it has been a bruising couple of days for the perceived frontrunner.