By Gram Slattery SIOUX CENTER, Iowa (Reuters) – Donald Trump used his first visit to Iowa this year to attack Republican competitor Nikki Haley and to hit back against Democratic President Joe Biden, who denounced him as a threat to American democracy earlier on Friday. Speaking to a crowd of several hundred supporters just 10 days before the crucial Iowa caucus – the first round of the Republican nominating contest – the former president presented a dark portrait of the United States. He called it a “failing” nation, beset by “terrorists” and immigrants from “mental asylums” pouring over the …