Me, Myself, and I
Andrew Johnson, who would succeed Lincoln as president.
If the President is impeached and convicted, the Vice President would succeed him. The Vice President would then assume the role of President for the remainder of the term.
Joe Biden, the Vice President
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The presidents spouse
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Joe Biden, the vice-president, would take over if Obama were to die,
Bill Clinton can not legally serve as Vice-president because he can not again be president, having already served two full terms. If Hillary were elected and died, the legally installed vice-president would succeed her.
bring the president votes in the election from a group or region that would not otherwise be a likely source of support.
Next in line is the Speaker of the House, who in 2016 is Paul Ryan.
political reform
The Vice President could become Acting President in the event of Presidential disability, and there is no codified term. A new Vice President would not be nominated. Should the Vice President succeed to the Presidency, then he would serve out the remainder of that President's term, and a new Vice President would ordinarily be nominated to fill the Vice President's unexpired term.