How Trump could be denied the presidency, even if he wins the election.
The 12th Amendment to the Constitution of the United States provides “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.“
It takes 270 Electorial votes to be elected president. Right now the race
is close. If the Libertarian Party candidate Gary Johnson were to win a
state such as Utah where Gary Johnson now has 13% or New Mexico where he
was Governor that could and probably would deny any candidate from
receiving a majority and throw the election into the House of
Representatives where many if not most Congressmen dislike Trump.
They will be told that if they do not abandon Trump and give all their
votes to Johnson, then Hillary will be President. In that scenario, Johnson
will almost certainly become president.
Another possibility is that Trump wins enough states to give him 270
electors but two or three of his electors defect and vote for another
Republican such as John Kasich. This could happen in Ohio where Kasich is
sitting governor. This could cause a stampede of Trump electors to vote for
Kasich instead because otherwise Hillary would be President.