The selection of the Vice President of the United States is undertaken by the president. When a candidate is running for presidential office, he selects a running mate who becomes the vice president when the candidate wins election.
Eligibility for the vice presidency is the same as that for the presidency. The chosen candidate must be 35 years of age or older and be a U.S. citizen who has resided in the country for at least 14 years.
Since potential vice presidents are chosen before the election occurs, they are often selected partially due to their potential effect on the election. Presidential candidates often choose running mates who can capture key demographics or who have a high level of experience in an area that voters regard as important. In many cases, top running mate candidates are popular in key states in a given election.
Most vice presidents have had significant legislative experience, and presidential candidates often look to complement their own areas of experience and expertise with those of their vice presidential choice. Running mates must be chosen based on their potential contribution to the executive office after the election, so each presidential candidate should consider the positive effects his vice presidential choice can bring to his political party, his country and himself.