There are few things that democrats enjoy more than finding sleazy ways to bypass provisions of the US Constitution.
That’s because, to date, so few of them have been shot while doing it.
A popular vote scheme would be vulnerable to fraud. Currently we have a decennial census to determine states' populations and assign electoral votes accordingly (along with senators). Since the Constitution recognizes the States power to control the election process a state wishing to commit fraud would simply create millions of multiple voter registrations and stuff the ballot boxes to overwhelm the national numbers. Under the current system states can commit all the fraud they can get away with but the damage is limited by the set number of electoral votes allocated to each state.
If you thought the Florida recount process was a circus, imagine if every state had to do a recount to make every vote count. Imagine how many additional ballot boxes would be uncovered.