The catch is the electoral college and the historical reason for its existence. If you
google “history of the electoral college” you will get lots of explanations of how and why it came to be. Most of them either leave out completely or treat too lightly, the real driving force behind the creation of the electoral college.
The electoral college originally set forth in Article II, Section 1, Clause 3, was immediately seen to create a mess. In the 1796 election John Adams became president and Thomas Jefferson the vice-president, although Adams’ running mate was Thomas Pinckney. Adams and Jefferson were bitter enemies at the time (though later became ardent friends and correspondents). To put this in perspective, if the 12th Amendment, ratified in 1804, had not changed the way the electoral college operates, Donald Trump would have won the presidency and Hillary Clinton might have won as vice-president-elect. [She would have needed to get more electoral votes than Mike Pence, however]. If that had happened, it would not have been exactly a tenable result.
The only part of the original electoral college preserved in the 12th Amendment is the rule that president and vice-president cannot both be from the same state. That’s why Dick Cheney had to abandon his residency in Texas and return to his home state of Wyoming in order to run as George Bush’s vice-president.
A simplified understanding of the 12th Amendment as it has operated since 1804 is that the candidate receiving a majority of the electoral votes is president and his vice-presidential running mate is vice-president. Only if no candidate receives a majority of electoral votes does the election get thrown into the House of Representatives. That happened in 1824 and not since.
There are a total of 538 electors, one for each member of the House of Reprentatives (435) and one for each senator (100), and 3 for the District of Columbia. Each state thus has one elector for each House seat and one for each senate seat.
Since 1880 all states except Maine and Nebraska choose electors on a “winner-take-all” basis. Maine and Nebraska allocate them according to the “congressional district method”, selecting one elector within each congressional district by popular vote and awarding two electors by a statewide popular vote.
There is no federal law restricting how electors vote but only rarely has an elector departed from voting according to the winning party for that state. This is because each of the two major parties can appoint their own electors. Only the most stalwart party supporters are ever chosen as electors for that party. The current effort by the Hillary Clinton campaign to get some Republican electors to switch their votes to Hillary has no chance of success.
So why did the founding fathers even want an electoral college? The original reason was to get the 13 colonies to ratify the Constitution. By 1780 five colonies (MA, CN,NY, PA, VA) held 55% of the total population, the rest was divided up among the other 8 colonies. By 1789 that ratio no doubt held and likely got even more skewed.
In order to get enough of the other 8 to agree to ratification the electoral college was created. In this way the less populace among the newly created states would be guaranteed some measure of Representation. The Electoral College was created to ensure that every state in the union has representation, and that remains an important factor today. Today the population of the United States is concentrated in populace cities, mainly on the East and West coasts. Without the electoral college there would be little reason for anyone in the flyover states to vote at all in presidential elections. The popular vote would invariably coalesce in the highly populated states and would elect the president in every election. If that had been the case in the last election Hillary would be President-elect with the support of only 11 states:
There is one more thing to consider, however. Without the electoral college the popular vote might have been different and Hillary might still have lost. Because of the electoral college and because the West Coast States and the Northeastern states are so solidly, one might say mindlessly Democrat, neither candidate wastes time or money campaigning in those states. It’s almost as if the people in those states are wasting their time and effort in voting because these each of these state always go to the Democrats (1984 was an exception when the election was between Ronald Reagan and Walter Mondale).
However, if there were no electoral college the Republican candidate would be compelled to campaign in those states, and the Democrat candidate would be compelled to defend their policies to the voters. The Republican candidate could therefore lose the popular vote in those states (albeit with a narrower margin) and still win the national popluar vote. If that had been the case in the 2016 election Donald Trump would surely have got more popluar votes in those states, and with only about 2 million votes to make up, he may very well have won the popular vote nationally.
What does this mean? It means that if the Democrats really want to get rid of the electoral college, and were successful in doing so, they could easily come to regret that decision. If states like California and New York became battlegrounds in which Republicans could actually benefit by getting votes in those states, even if less than a majority in those states, it could become harder for Democrats to win national elections.