Hitler as President of the US? Interesting...
OK, so Hitler was born in April 1889 which means he was conceived somewhere around July 1888 if my calculations are correct. For him to become President. Something has to happen after July 1888 and before April 1889 for them to migrate to the US so Adolf Hitler to become President. I'd like to propose a mini-TL:
In September 1888 Alois Hitler is fired from his job as a customs official due to a younger and more ambitious competitor for his leading position, thus making him unemployed. He decides to take his small fortune, which he earned through years of diligent work and saving money, to the United States. Alois Hitler and his wife Klara arrive in November 1888. Adolf Hitler is born in April of the following year. While his father ignores him mostly, he is doted on by his mother because he's such a sickly baby.
Adolf Hitler along with his family settle down among the Austrian community of New York in Palenville and at a young age he's enrolled into a primary school where he learns English in which he's relatively proficient although he fails to lose a small accent. He does show leadership skills as he gathers a small clique of powerful friends around him which makes him influential on the playground. He's otherwise an unremarkable student in the eyes of his teachers. He claims to 'defend' his German and Austrian friends from bad influences.
After primary school he cannot get secondary education because his father was unable to fund it since he couldn't get a job due to a lack of command of the English language. Young Adolf starts to work in a cleaning job in a German 'sauerkraut and wurst' restaurant and his smooth tongue and competitive skills allow him to work himself up to the position of financial assistant to the owner, after having self-studied US financial law. This period of competition with a number of other more qualified pretenders form his attitude toward life: everything could be overcome by will power and strength. He switched from a number of lower administrative positions in a number of small and medium businesses using his competitive skills.
In the early 1900s decides that the commercial life is nothing for him, he wants to prove himself. He decides to join the army to prove his loyalty to the Unites States, the only fatherland he has ever known besides Austria which he had heard only heard from in stories from his parents and to which he had already developed a disliking. He joins the army in 1908, but doesn't rise above the rank of sergeant in spite of his diligence.
In 1915 he's involved in the US occupation of Haiti and his promoted to sergeant major for "valorous efforts against destabilizing forces". In 1917, the US gets involved in WW I and Adolf Hitler serves in the American Expeditionary Force and rises to the rank of lieutenant owing to courage and ability to lead under fire. He fought in the German Spring Offensive and the first US offensive, the Battle of Cantigny in May 1918. He's wounded in the latter and becomes a staff officer.
After the end of WW I in November 1918, Adolf Hitler isn't demobilized like many others and remains a staff officer for a few more years. He then goes into reserve and returns to his old neighbourhood to find a lot of discrimination against German and Austrian Americans. He goes into local politics and proves to be an extremely gifted orator and gains a following among a number of immigrant groups due to his egalitarian rhetoric and talks about economic, social and educational reform.
He is subsumed into the more progressive Democrat party and shortly hereafter he changes his name to Arthur Hitler. He rises through the ranks from the local government as a member of city council in Palenville to district attorney for New York in 1925 and succesfully becomes a member of Congress in 1928. His rhetoric is immensely popular and he becomes a prominent party member in a few years using radio, newspapers and other media with New York as his home base. He wins the Democrat ticket for the 1932 Presidential elections.
Against all odds, he wins. His strong government intervention, banking reform and Keynesian policies have effect and he's easily re-elected in 1936 and 1940. He marries a younger American woman in 1933, a late marriage. In the meantime, new conflict erupts in Europe between a resurgent militarist Germany on one side and France and Britain on the other with a neutral USSR and Italy. Japan gets involved and attacks Asian colonies, including the Philippines.
President Hitler declares war. Although the first battle against Japanese carriers is no success, the US gains the upper hand in the Asian theatre and later in the European theatre as well, dropping atomic bombs on Hamburg and Munich in 1945. Hitler is involved in the planning of the war. He's personally present in Tokyo and Berlin to accept the instrument of surrender of both countries. He wins the 1944 Presidential election and serves out a fourth term, but amid popular opposition he decides to decline running for a fifth term in the 1948 presidential elections.
He becomes a political talking head and remains influential in the Democrat party into the 1950s. He formally retires in 1955 and takes up amateur painting as a hobby while writing his memoirs. He dies in 1960 at the age of 71, leaving a 26 year old son named John Hitler who is now a prominent senator.