"The tipping-point state is the first state that gives the winning candidate a majority of electoral votes, thereby securing the candidate's victory in the Electoral College, when all states are arranged in decreasing order of their vote margins for the ultimate winner."

https://en.wikipedia.org/wiki/Tipping-point_state

Resolves as ambiguous if the election goes to the House because no candidate has a majority in the Electoral College. Margin is determined by percentage, not absolute vote count.