The first thing is that he was not personnally elected president of the Weimar Republic. He tried but everybody else voted for Hindenburg. The Nazi party won the elections and a coalition was almost impossible without him...well they thought. So Hindenburg made him chancellor in a coaltion government with the conservatives, lead by Von Pappen. This is the key element. Almost nobody realized the full significance of this designation because everybody thougt (including Von Pappen himself) that he was under control with the conservatives to "control him". They realized too late their mistake. Hitler had now the state with him to control the next election and in addition the Reichtag fire gave him the powers he needed to control the state. When the great, old marshal Von Hindenburg died Hitler became president and chancellor at the same time, finishing the process.