美国总统奥巴马（Obama）已进入任期的最后一年。他最近开玩笑说，他的一些支持者希望他能继续留任。2015年11月，他在纽约对民众发表讲话时说：“他们会说，巴拉克，希望你能再参加一次竞选。”

他说：“但我解释说：第一，这是违反宪法的。乔治·华盛顿(George Washington)树立了一个好榜样；第二，即使符合宪法，米歇尔（Michelle）也不会允许；第三，这从来都不是关乎我个人的事。”

所有的美国总统，除一人外，都效仿华盛顿在第二个四年任期到期后离开总统职位。1951年批准的美国宪法第22修正案(The 22nd Amendment to the Constitution)禁止任何人当选总统超过两次。

1912年的一幅政治漫画描绘了乔治·华盛顿的幽灵劝阻前总统西奥多·罗斯福(Theodore Roosevelt)寻求第三个总统任期。(National Archives)

有些人不赞同任期限制。他们认为，选民应该能够保留自己喜欢的总统，同时任期限制也削弱了总统在第二个任期的能力，成为所谓的“跛脚鸭”。

但任期限制有助于推出具有新思维的新候选人，也可防止人们认为某一位领导人是不可或缺的想法。支持者认为，总统职位是一份工作，而不是一个职业。

科罗拉多学院（Colorado College）以总统任期为研究课题的教授托马斯·克罗宁(Thomas Cronin) 说，“第22修正案算不上很完美，但也是一个很实际的妥协方案，一来考虑到担任总统一职需要有精力、决断能力和领导才干，二来也顾及了轮换执政的共和原则。”

三分之二的美国人支持第22修正案对总统任期的限制。克罗宁说：“大多数美国人都了解，任期限制是为了保护自由不得不在权衡之下做出的取舍。”

Now in his last year in office, President Obama recently joked that some of his supporters would like him to stay on. “They’ll say, oh, Barack, I wish you could run another term,” hetold a New York audience in November.

“But I explain: A, it’s unconstitutional. George Washington set a good example. B, Michelle would not permit it, even if it were constitutional. And, C, this has never been about me,” he said.

All but one of the U.S. presidents followed Washington’s example and walked away from power after their second four-year terms. The 22nd Amendment to the Constitution, ratified in 1951, now prohibits anyone from being elected president more than twice.

A 1912 political cartoon shows former President Theodore Roosevelt being discouraged by George Washington’s ghost from seeking a third presidential term. (National Archives)

Some disagree with term limits. They argue that voters should be able to retain a president they like. Term limits also weaken second-term “lame duck” presidents, they contend.

But term limits encourage new candidates with fresh ideas and guard against the notion that any single leader is indispensable. The presidency is a job, not a career, proponents argue.

Thomas Cronin, a professor at Colorado College and expert on the presidency, says, “The 22nd Amendment [imposing presidential term limits] is a practical if imperfect compromise between the need for energy, decisiveness and leadership in the presidency and the republican principle of rotation in office.”

Two-thirds of Americans support the 22nd Amendment and presidential term limits. “Most Americans understand that term limits are a trade-off for the protection of liberty,” Cronin said.