President Barack Obama is asking voters to elect Democrat Hillary Clintonor risk dangerous policy changes by Republican Donald Trump.But Clinton, who made history as the first womanto win a major party presidential nomination, faces a historical problem.Only once since 1960 has a two-term president been replaced by a candidate from the same party.Democrat Clinton is trying to succeed Democrat Obama,who ends his 2nd term as president on January 20, 2017.Clinton is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 Thursdayat 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 partybecause voters often want change, said Matthew Kerbel.He is the chair of the political science department at Villanova University in Pennsylvania."It will be tough for Hillary Clinton to make a credible argumentthat she will bring a change in direction," Kerbel said."What Hillary has going for her is that Donald Trump hasn't yet sold the nationon the type of change he would bring if elected president."The one time in the last 56 years a candidatesucceeded a two-term president of the same party was in 1988.Republican George H.W. Bush beat Democrat Michael Dukakisto succeed Republican President Ronald Reagan.Bush was Reagan's vice president and some called his election, "Reagan's third term."All other efforts since 1960 failed.In 1960, Republican Richard Nixon lost to Democrat John F. Kennedy,as he tried to succeed two-term Republican President Dwight Eisenhower.In 1968, Democrat Hubert Humphrey lost to Republican Nixonto succeed Democrat Lyndon Johnson.Johnson served the final 14 months of Kennedy's first termand then four more years after his election in 1964.Nixon won a 2nd term in 1972.Republican Gerald Ford, who became president after Nixon resigned in 1974,lost the 1976 presidential election to Democrat Jimmy Carter.In 2000, Democrat Al Gore lost to Republican George W. Bush.Gore was trying to succeed Democrat Bill Clinton,Hillary's husband, who served two terms as president.Gore was Clinton's vice president.In 2008, Obama defeated Republican John McCain.McCain was hoping to replace George W. Bush.McCain asked Bush not to campaign for him.Bush was unpopular because of a major economic collapse in 2008and an unpopular war in Iraq.At the convention on Tuesday night,all speakers talked about Hillary Clinton's historical achievement but one – Bill Clinton.The former president had this to say about his wife:"She's the best darn change-maker I've ever met in my whole life."Clinton has said she wants Obama to campaign for herand Obama has said he will do so often between now and the Nov. 8th election.A Washington Post/ABC News poll last monthsaid 56 percent of Americans view Obama favorably,his highest rating in five years.Obama beat Clinton for the 2008 Democratic presidential nomination.But he said, "there has never been a candidate better prepared for the presidency than Hillary Clinton."The president also criticized Trump for failing to "spend a lot of time trying to find out about issues,"such as U.S. nuclear defense options,or the difference between Sunni and Shia Muslims.Trump responded Wednesday by saying it is Obama and Clinton who are not prepared to lead the country.On Wednesday, he called Obama "the most ignorant president in our history" and "a disaster.""And I believe Hillary Clinton is even worse," he said.