President Barack Obama is asking voters to elect Democrat Hillary Clinton or risk dangerous policy changes by Republican Donald Trump. But Clinton, who made history as the first woman to win a major party presidential nomination, faces a historical problem. Only once since 1960 has a two-term president been replaced by a candidate from the same party. Democrat Clinton is trying to succeed Democrat Obama, who ends his 2nd term as president on January 20, 2017. Clinton is scheduled to accept the Democratic presidential nomination on Thursday at the party’s convention in Philadelphia, Pennsylvania. Trump, a wealthy businessman, accepted the Republican nomination in Cleveland, Ohio, last week. It is difficult to succeed a two-term president of the same party because voters often want change, said Matthew Kerbel.