What likely scenarios are imaginable leading to a Constitutional amendment in the US eliminating the Electoral College system and instituting popular election of the President?
Amending the US Constitution is a difficult, weighty procedure.
Article Five of the Constitution addresses amending the highest law document in the US. The simplest way to amend the Constitution and the only way that’s been used to date is to pass the amendment by ⅔ vote in both houses of Congress and then obtaining passage in ¾ of legislatures of the various states. (A more detailed explanation can be read here)
In my opinion, the Electoral College is an eighteenth century noose around the US’s neck, and it should be eliminated. Three times, it has led to the candidate who won the popular vote being denied the Presidency. Most will remember that happening in the election of 2000.
What might be the impetus for successfully amending the Constitution in this matter?
If you wish to discuss whether the Electoral College should be eliminated at all, please, ask a separate question.