Mitt Romney is the favourite to become the Republican Party's candidate in November's US presidential election. But in what could be a tight race with Barack Obama, Romney's Mormon faith might be a deciding factor.While the majority of US voters will be unconcerned about Romney's religious beliefs, a significant minority say that they would think twice about voting for a Mormon as president. Evangelical Christians are especially concerned, with some believing that Mormonism is more a cult than a mainstream Christian creed. Others fear a Romney administration would be unduly influenced by the church's attitude to such matters as polygamy, gay rights and abortion.So what is Mormonism and what would a Mormon in the White House mean for the US and the world?