Withing 20 years, 2 Democrats have won the popular vote but lost the US general elections because of how the Electoral college works. Democrats get the votes from population concentrated in urban areas of East and West coast but the 'cut off' heartland is where they lose. Supporters of Electoral College speak about the need to give voice to those who are in minority - i.e. states with lesser but angrier populations, who may also resent that without the Electoral college, big cities would drown out their voices.
The Electoral college in summary: Each state has a set number of electors that is based on the state's population -- the candidate who wins the state's popular vote gets those electors. So, on Election Day, the American people are electing the electors who elect the president.
1. Problem #1 with the Elecoral college:
Small states have outsized representation in the electoral college, as compared to their population. Wisconsin has a population of 5.8 million and 10 electoral votes. Texas has a population of 27.0 million and 38 electoral votes. If you won 5 states the same size as Wisconsin, it would represent a population of 29.0 million and you'd get 50 electoral votes. Texas has a population that is 7% smaller than 5 Wisconsins, but is allocated 24% fewer electoral votes than 5 Wisconsins would have.
Opponents of the Elecoral College also point out to the undue importance of Swing states, with fewer populations abut where the Democrats and Republicans are evenly (mor eor less) matched, and even a very small win margin gives all the votes to the winner.