What would happen if no one ran for president?

[WillK]

As far as the Constitution is concerned, no one votes for President on Election Day, rather the State Legislatures have chosen to allow them to vote for Electors. If for some strange reason, all 538 Electors nominate different people, then it would be a wide open race in the Congress with the House having 538 choices for President and the Senate having 538 choices for Vice President.

Isn't it limited to only the top three?

Technically they're all tied for first if every elector nominated a different person, so that rule isn't relevant.

The 12th amendment does not account for a tie any diffrently, so the house would have to pick 3 to decide from.

[WillK]

As far as the Constitution is concerned, no one votes for President on Election Day, rather the State Legislatures have chosen to allow them to vote for Electors. If for some strange reason, all 538 Electors nominate different people, then it would be a wide open race in the Congress with the House having 538 choices for President and the Senate having 538 choices for Vice President.

Isn't it limited to only the top three?

Technically they're all tied for first if every elector nominated a different person, so that rule isn't relevant.

The 12th amendment does not account for a tie any diffrently, so the house would have to pick 3 to decide from.

That is absurd. The House would pick three for itself to decide from - so a two-round vote between all 538? Where do you believe to be drawing that from, exactly?

12th Amendment says: "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."

The House would only be abel to choice from 3.  How the 3 would be picked is left unsaid and, if there was a tie involving more than 3, deciding that would probably cause a crisis in Congress.

[WillK]

The most straightforward reading of the clause you quote is that, if four or more persons are tied at the top, then nobody is eligible to be chosen by the House.

I disagree.  If there is a four way tie, then "from the persons having the highest numbers"  there can still be three to pick from who have that highest number, with the fourth left out.