President Barack Obama is asking voters to elect Democrat Hillary Clinton or risk dangerous policy changes by Republican Donald Trump.
美国总统奥巴马让选民选择民主党人希拉里·克林顿（Hillary Clinton）或是冒共和党人唐纳德·川普（Donald Trump）上台导致政策变化的风险。
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Only once since 1960 has a two-term president been replaced by a candidate from the same party.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.
Voters want change
It is difficult to succeed a two-term president of the same party because 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 argument that she will bring a change in direction," Kerbel said. "What Hillary has going for her is that Donald Trump hasn't yet sold the nation on the type of change he would bring if elected president."
The one time in the last 56 years a candidate succeeded a two-term president of the same party was in 1988. Republican George H.W. Bush beat Democrat Michael Dukakis to succeed Republican President Ronald Reagan. Bush was Reagan's vice president and some called his election, "Reagan's third term."
过去56年发生过一次候选人继任同一党派连任两届的总统，那是在1988年。当时共和党人老布什（George H.W. Bush）击败民主党人迈克尔·杜卡基斯（Michael Dukakis）接替了共和党人总统罗纳德·里根（Ronald Reagan）。老布什曾是里根的副总统，有人称他的当选是里根的第三任期。
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.
1960年，共和党人理查德·尼克松（Richard Nixon）试图接替连任两届的共和党人总统德怀特·艾森豪威尔（Dwight Eisenhower），但是败给了民主党人约翰·肯尼迪（John F. Kennedy）。
In 1968, Democrat Hubert Humphrey lost to Republican Nixon to succeed Democrat Lyndon Johnson. Johnson served the final 14 months of Kennedy's first term and then four more years after his election in 1964. Nixon won a 2nd term in 1972.
1968年，民主党人休伯特·汉弗莱（Hubert Humphrey）在接任民主党人林登·约翰逊（Lyndon Johnson）上败给了共和党人尼克松。约翰逊接替了肯尼迪第一个总统任期内的最后14个月，然后1964年当选再次获得四年任期。尼克松在1972年赢得了第二个总统任期。
Republican Gerald Ford, who became president after Nixon resigned in 1974, lost the 1976 presidential election to Democrat Jimmy Carter.
1974年，共和党杰拉尔德·福特（Gerald Ford）在尼克松辞职后成为总统，但是在1976年的总统选举中败给了民主党人吉米·卡特（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.
2000年，民主党人阿尔·戈尔（Al Gore）败给了共和党人小乔治（George W. Bush）。戈尔当时正试图接替任满两届的比尔·克林顿（Bill Clinton），也就是希拉里的丈夫。戈尔是克林顿的副总统。
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 2008 and 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."
Obama to campaign for Clinton
Clinton has said she wants Obama to campaign for her and Obama has said he will do so often between now and the Nov. 8th election. A Washington Post/ABC News poll last month said 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.