The battle for the Republican presidential nomination turned increasingly nasty as favorite Mitt Romney faced last-ditch attempts from rivals desperate to forestall his victory march. An attack ad depicting Romney as a heartless corporate raider set the tone for what promises to be a bitter January 21 primary contest in South Carolina, a state that is no stranger to the dark arts of US electioneering. The 28-minute video, "When Mitt Romney Came to Town," features interviews with people claiming Romney "destroyed" them during his reign at private equity firm Bain Capital, breaking up companies while reaping millions in profits. The ad is being run by a campaign group aligned with Newt Gingrich, the former House speaker who is out for revenge after Romney torpedoed his chances in Iowa with a late negative ad blitz en route to a narrow win. After a landslide victory on Tuesday in New Hampshire, Romney is strongly favored to claim the Republican nomination and take on Democratic President Barack Obama in the November 6 general election. But the frontrunner will face stiffer resistance as he heads south into more conservative territory, especially as rivals see South Carolina as their last realistic chance to derail his campaign. Evangelical Christians constitute a majority in the state and many will be wary of the former Massachusetts governor's Mormon faith and his flip-flopping on hot-button conservative issues such as abortion and gay rights. Romney hopes that Gingrich, former Pennsylvania senator Rick Santorum and Texas Governor Rick Perry will split the core conservative vote, allowing him a path to a third consecutive victory and a virtual lock on the nomination. Romney also has the key endorsement of South Carolina Governor Nikki Haley. The last polls for South Carolina show Romney on about 30 percent, clear of Santorum and Gingrich on around 20 percent. The Republican establishment appears increasingly resigned to the fact that Romney will be the nominee and some senior figures were urging the likes of Gingrich not to go too negative in the campaign.  "I would say, Newt, you're a great American, and get back on your positive focus, talk about your big ideas," South Carolina Senator Jim DeMint told the Politico website, referring to an anti-Romney abortion ad. "I don't think that's healthy right now... This idea of condemning people who change their minds is not a good idea." Romney says his private sector triumphs and business acumen make him the best person to take on Obama, whose reelection bid is weighed down by the sagging US economy and high unemployment. Knockout wins in South Carolina and in Florida on January 31 could seal him the nomination, although the campaign schedule prevents him being formally endorsed before April. "I have a good start. I have an uphill climb in South Carolina ahead of me, but it could not have worked better last night," Romney told NBC on Wednesday. South Carolina has a notorious reputation for toxic campaigning and its conservative demographics pose a hurdle for the frontrunner. The most infamous and high-profile attack in the state was scripted by George W. Bush's 2000 presidential campaign, which suggested that White House rival Senator John McCain had fathered a baby with a black prostitute. It was also suggested that the Vietnam veteran was brainwashed during his time as a prisoner of war and that his wife was addicted to prescription drugs. Bush denied any involvement in the smears, but McCain lost in the South Carolina primary and never recovered in his first bid to become president. Despite romping to a win in New Hampshire with 39 percent of the vote, becoming the first Republican non-incumbent in the modern era to win the first two contests, Romney is still struggling to unite his party behind him. Veteran Congressman Ron Paul of Texas, a small-government champion, came in second in New Hampshire and former US envoy to China Jon Huntsman, who bet all on a strong showing in New Hampshire, trailed in third. Romney's staunchest opponents fall broadly into two camps -- ultra-conservative Tea Party groups yearning for a pure small-government candidate, and evangelicals who doubt he shares their core convictions. About 150 evangelical leaders are set to gather at a Texas ranch on Friday to plot a path away from Romney, and the Washington Post reported that Gingrich and Santorum plan to address a Tea Party convention this weekend in Myrtle Beach, South Carolina. Santorum, who lost out by just eight votes to Romney in Iowa, finished fifth just behind Gingrich in New Hampshire, but both see South Carolina as far more fertile ground for their conservative message.