If the President of the US cannot fulfill his or her term due to things like death, illness - physical or mental, disease or other factor, the President can resign from office whereupon the Vice President becomes President. Also, Amendment XXV (25) to the Constitution provides for circumstances where the Vice President becomes the Acting President.
I encourage you to read Amendments XII and XX also as they also deal with presidential succession.
The vice president serves as president if the president is unable to function.
'the speaker of the house'
He was killed by John Wilkes Booth during his 2nd term as president.
The Speaker of the House would become president if the president and VP were incapacitated at the same time.
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they vote for a new one
us presidents are allowed to sever two terms only
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Barack Obama is our president but not for long his term is coming to an end and he wont be able to serve another term do to the fact that he has served all his terms and cant run for presidency again