How to prevent a Trump presidency

Hillary Clinton, who won 228 electoral-college votes, persuades her electors to vote for, let’s say, Mitt Romney. Then 42 of Trump’s 306 electors are persuaded to do the same. Romney would then have 270 electoral-college votes and would be the president.
Or if no one ends up with 270 votes, the election will be decided in the House of Representatives. There, a coalition of Democrats and anti-Trump Republicans could make Romney president.
Why would so many Democrats vote for Romney? Because they believe as I do that Donald Trump is so evil and dangerous that we must be willing to make any compromise necessary to keep him out of the White House.
Eric Bahrt