Answer:
The Constitution assigns the Vice President to preside over the Senate with the right to vote in case of a tie vote. Also, Amendment 25 gives him a role in deciding the question of presidential disability. Of course , he also has a duty to be prepared to take over as President if the president becomes unable to serve/
Article I, Section 3, Clause 4 "The Vice President of the United States shall be President of the Senate, but shall have no vote, unless they be equally divided."
Article II, Section 1, Clause 6"In case of the removal of the President from office, or of his death, resignation, or inability to discharge the powers and duties of the said office, the same shall devolve on the Vice President...
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