Barack Obama on Monday sought to portray himself as a mainstream American patriot and put his Republican opponents on notice that he would “not stand idly by” when others questioned his support for the US flag.

Mr Obama’s speech, which he delivered in Independence, Missouri, the birthplace of Harry S. Truman, the former president who was known for his unassuming Mid-Western style of patriotism, comes after months of pointed attacks on his biographical credentials.