Twelfth Amendment
Tie scenario
If Vice President Harris wins only Pennsylvania, Michigan and Wisconsin the outcome will be a 269-269 tie in the Electoral College. This assumes that Harris does not win Georgia, North Carolina, Arizona or Nevada, each of which Trump wins. Otherwise, every remaining state is won by the candidate who won it in 2020.
Without the Nebraska Second
The models assume that except for the seven swing states, each candidate will win the states and election districts won in 2020. For Harris, the total includes a Nebraska district that centers on Omaha. Nebraska is one of two states that have this arrangement. The other is Maine. An attempt was made earlier this year to change Nebraska to the winner-take-all system used by other states. It was unsuccessful, and it is not clear if another attempt will be made. It could be added to a special session anticipated for July to deal with property tax relief, but as of mid-June there has been a scarcity of news raising this. There also appears to be some internecine conflict going on between the Congressional delegation and the MAGA-aligned party apparatus. Should winner take all be implemented or if the Nebraska Second this time votes for Trump, possible outcomes differ.
A tie outcome that was produced by Harris winning only Wisconsin, Michigan and Pennsylvania, now has three different possible scenarios
Wisconsin, Georgia and Pennsylvania
Wisconsin, Pennsylvania and North Carolina (through May, NC is not moving in Harris's direction)
Arizona, Michigan and Pennsylvania
Those three scenarios, which were formally wins, are replaced as the smallest possible victories by
Nevada, Wisconsin, Arizona and Pennsylvania
Arizona, Georgia and Pennsylvania
Arizona, Pennsylvania and North Carolina
Constitutional framework for a tied vote
The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed; and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice.
U.S. Constitution, Twelfth Amendment [emphasis added]
Current situation
A majority of 50 state delegations is 26, which is currently what the Republican Party controls. In addition, Maine, Minnesota and North Carolina have equally divided delegations. The remaining 20 delegations are controlled by the Democratic party.
Following the presidential and congressional elections
If the Republican Party retains control of 26 states, it is in a position to choose the Vice President.
If the Republican Party loses control of one state, no party has sole power to choose the Vice President.
In that case, the provisions of the Presidential Succession Act come into operation.
Should a majority of states fail to agree on the selection of the President and also fail to agree on the selection of a Vice President, the Speaker of the House will become President on Inauguration Day or, if there is no Speaker, the President Pro Tem of the Senate. If there should be no President Pro Tem of the Senate, the Secretary of State would become President.