Under what circumstances must Congress choose a Vice President?

Because of the 12th amendment the house of representatives may be called on to elect a president. if no candidate receives a majority of the electoral votes for president, the house of representatives, voting by states, must decide the issuing that situation, it must choose from among the 3 highest contenders in the electoral college balloting. Each state has but one vote to cast, and a majority of the states is necessary for election.