Under the Constitution, the vice president serves as President of the Senate. He or she may vote in the Senate (ex officio, for he or she is not an elected member of the Senate) in the case of a tie, but is not required to. For much of the nation's history the task of presiding over Senate sessions was one of the vice president's principal duties (the other being to receive from the states the tally of electoral ballots cast for president and vice president and to open the certificates "in the Presence of the Senate and House of Representatives," so that the total votes could be counted). Since the 1950s, vice presidents have presided over few Senate debates. Instead, they have usually presided only on ceremonial occasions, such as joint sessions, or at times to announce the result of significant legislation or nomination, or when a tie vote on an important issue is anticipated.\n\nBased on this passage, when does the president of the senate vote?
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To answer this question, we should know that: He or she may vote in the Senate (ex officio, for he or she is not an elected member of the Senate) in the case of a tie, but is not required to.
So, the answer is in the case of a tie.
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