Maine and Nebraska are the only two states that have allowed, by state legislative action, a split in the electoral vote for President.
Maine adopted this concept in 1972 and Nebraska in 1992, and a split has occurred once in each state.
In 2008, Barack Obama won the 2nd district of Nebraska (Omaha and its suburbs), the first and only time that Nebraska has seen a Democratic electoral vote since 1964.
In 2016, Donald Trump won Maine’s 2nd district, which covers most of the state away from Portland, Augusta, and nearly coastal areas, with that being the first time a Republican won an electoral vote since 1988.
So if Donald Trump won every state he gained in 2016, except Michigan and Pennsylvania, he would win the Electoral College no matter what the popular vote majority of the Democratic Presidential nominee, by a 270-268 margin. But if he lost the 2nd district of Maine, the Electoral College would be 269-269.
The same would occur if the Democratic Presidential nominee won the 2nd district of Nebraska as Obama did in 2008, as then the Electoral College would be 269-269.
This would be a true constitutional crisis beyond any other Presidential election in American history!