It's worth noting that the same thing happened to the Democrats in 2000, and hadn't happened in the US at that point for over 100 years.
Looked at from the perspective of the Democratic party: they have won the popular vote 4 out of the last 5 elections, but only won the presidency twice. It's not surprising the party's frustration with the Electoral College would continue to grow.
https://en.wikipedia.org/wiki/United_States_presidential_ele...