Republican presidential candidates have begun the final TV debate in Iowa before the state\'s keenly anticipated nominating contest next month. With seven candidates remaining, former House Speaker Newt Gingrich has been surging in Iowa\'s opinion polls. But he is under attack from close rival, former Massachusetts Governor Mitt Romney. The eventual Republican nominee will challenge Democratic President Barack Obama in next November\'s elections. Thursday\'s forum, hosted by Fox News in Sioux City, Iowa, began at 21:00 EST (02:00 GMT). Also at the debate are Texas Republican Ron Paul, Minnesota Congresswoman Michele Bachmann, Texas Governor Rick Perry, former Utah Governor Jon Huntsman and former Pennsylvania Senator Rick Santorum. The campaign of Mr Romney, long viewed as a front-runner, has assailed Mr Gingrich\'s conservative credentials and leadership style in recent weeks. In an eve-of-poll interview with the New York Times, Mr Romney called the former House Speaker \"zany\" for having endorsed mining the moon and lighting highways with mirrors in space. Former Speaker of the House Newt Gingrich 15 December 2011 Newt Gingrich earned a reputation for aggressive partisanship during his time as House Speaker On 3 January, Iowa will hold the first in a series of state-by-state non-binding votes, known as primaries and caucuses, to pick a Republican nominee. The eventual candidate will be officially declared at the national party convention next August. Mr Gingrich is the latest front-runner, following the rise and fall of Mrs Bachmann, Mr Perry and Georgia businessman Herman Cain, who bowed out earlier this month amid allegations of sexual harassment and an extramarital affair. The former House Speaker has also been powering ahead in opinion polls from South Carolina and Florida, both early-voting states, too. Pivoting to face the rise of Mr Gingrich, Romney surrogates have lambasted the former House Speaker, whose own backers accused the onetime governor of a panic attack. Mr Gingrich\'s latest ad makes an effort to remain above the fray while still portraying his rivals as negative. \"Others seem to be more focused on attacks rather than moving the country forward,\" says the former House Speaker, who has pledged to run a positive campaign. \"That\'s up to them.\" However, earlier this week Mr Gingrich called on Mr Romney to pay back all the millions he earned running a private equity firm \"bankrupting companies and laying off employees\". The former House Speaker has been assailed for earning millions of dollars in consulting fees from the healthcare industry and federal housing agency Freddie Mac before the economic downturn and home foreclosure crisis. Gov Perry\'s new attack ad, meanwhile, seeks to portray both Mr Gingrich and Mr Romney as \"political insiders\", while Mrs Bachmann lumped the two front-runners together as \"Newt Romney\" during a recent debate. Mr Romney is viewed as the favourite of the Republican party establishment because opinion polls suggest that he has the best chance of beating Barack Obama, but the former Massachusetts governor has failed to electrify conservatives.