What is the essential, core, most important problem with the Electoral College system?
Here are some answers that I do not think are accurate:
The votes in every state are not counted equally in choosing the national leader
The votes in low population states have greater weight in choosing electors and this is unfair to voters in high population states
The candidates only visit swing states
These are true statements. But the core problem is that the system does not oblige the parties to compete for every vote in every state; instead, they can take the outcome for granted in at least 40 if not more states. More than 80% of voters and citizens live in the land of the ignored.
In the 40 or so taken-for-granted states the parties do not work as hard as they could to register voters, get out the vote, promote access to polling places, lobby for voting by mail, fight for ex-felons to vote, or take any of the other steps that would promote participation in the election. Citizens see that the system does not seek their votes and that their votes do not matter. So millions of people do not participate in the general election. The total number of non-participants ranges between 15 and 70 million.
Voting is an important act in the creation of a common culture, a widespread sharing of beliefs and values. A system that does not seek to involve tens of millions of citizens is inclined to derogate the importance of those people and to breed in them a sense of resentment toward the rest of society.
Furthermore, an executive whose victory depends on a bare plurality of a handful of states – the current situation – has much less incentive to govern in response to the wishes of the majority of citizens. Not only in a democracy, but in any form of government, both fairness and utilitarianism dictate that government should aspire to serve the interests of most citizens. A method of choosing the president that requires the aspirants to appeal to most people is much more likely to produce solutions to collective problems – like how to address climate change or pay for public goods like education and transportation or provide health care insurance.
So non-participation and non-responsive government are the two aspects of the core problem with the Electoral College system.
Suppose that there were many more than a handful of swing states. Let us imagine that the top 10 states in population, with more than 50% of citizens in aggregate, were all swing states. Then the parties would have to seek every vote in these populous states. It would still be true that small population state voters picked more electors per resident than would be the case in the populous states. But if participation went up 10 to 30 million, concern about inequality of voting power between small population states and big population states would not be so terribly important.
The problem with the system in this century is that now and in the future states with most people are not contested.