“Campaigning for the Popular Vote is much easier & different than campaigning for the Electoral College,” Mr Trump tweeted on Tuesday evening.
“It’s like training for the 100 yard dash vs. a marathon. The brilliance of the Electoral College is that you must go to many States to win.”
The president’s outburst came after Elizabeth Warren, a leading Democratic contender for 2020, called for a popular vote to replace the complex Electoral College system, in which a president is selected by “electors” that people in each state vote for.
"Come a general election, presidential candidates don't come to places like Mississippi, they also don't come to places like California or Massachusetts, because we're not the battleground states," Ms Warren said at a Mississippi town hall on Monday.
"My view is that every vote matters, and the way we can make that happen is that we can have national voting, and that means get rid of the Electoral College," she said to applause.
Despite Mr Trump’s claim “you must go to many states” to win the Electoral College, he in fact campaigned at fewer than half of America’s states during the 2016 presidential election, but won 304 of the 538 available votes.
Campaigning for the Popular Vote is much easier & different than campaigning for the Electoral College. It’s like training for the 100 yard dash vs. a marathon. The brilliance of the Electoral College is that you must go to many States to win. With the Popular Vote, you go to....— Donald J. Trump (@realDonaldTrump)March 20, 2019
His win came despite losing the popular vote to Hillary Clinton by almost 3 million votes, a fact Mr Trump has appeared reluctant to admit.
“With the Popular Vote, you go to just the large States - the Cities would end up running the Country,” Mr Trump continued. “Smaller States & the entire Midwest would end up losing all power - & we can’t let that happen.”
In fact, a popular voting system would ensure all citizens’ votes were given equal weight, whether west coast or east, rural or urban.
There is no suggestion people in cities would wield disproportionate power beyond the fact a vast majority of Americans now live in urban areas.
“I used to like the idea of the Popular Vote, but now realize the Electoral College is far better for the U.S.A,” Mr Trump added.
Mr Trump’s change of heart appears to have occurred in the months since he lost the popular vote to Ms Clinton.
In 2012, when Mr Trump mistakenly believed former president Barack Obama received fewer votes than Mitt Romney, he branded Mr Obama’s presidential victory a “disaster for democracy” and called for a “revolution” in a series of tweets, most of which have since been deleted.
We can't let this happen. We should march on Washington and stop this travesty. Our nation is totally divided!— Donald J. Trump (@realDonaldTrump)November 7, 2012
In the early hours of Wednesday morning, Mr Trump tweeted: “The Democrats are getting very ‘strange.’ They now want to change the voting age to 16, abolish the Electoral College, and Increase significantly the number of Supreme Court Justices. Actually, you’ve got to win it at the Ballot Box!”
Nancy Pelosi, Democratic House speaker, recently spoke out in favour of lowering the voting age to 16, telling reporters voting rights would help teenagers who are “learning about government”.
A number of Democratic presidential hopefuls have also backed judicial reform, in the wake of growing Republican dominance of the US courts.
Kamala Harris and Ms Warren have both said if elected they would consider expanding the Supreme Court, which has seen a sharp turn to the right under Mr Trump.
It comes after congressional Republicans in 2016 blocked Mr Obama’s Supreme Court nominee Merrick Garland; an unprecedented move that allowed Mr Trump a year later to select Neil Gorsuch, a far more conservative judge.
How does the Electoral College work?
The Electoral College is the formal body that elects the president and vice-president of the US. The Founding Fathers established it in the Constitution as a compromise of sorts between having the president be elected by Congress or by a popular vote of the people.
On election day, when US citizens cast their vote, they are actually voting for the electors chosen by the candidates’ parties. Most states have a “winner-take-all” system that awards all electors to the winning candidate. Maine and Nebraska employ a type of proportional system, but since they only account for nine electoral votes in total, this has not influenced any election.
There are 538 electors that make up the Electoral College and a majority of 270 is needed in order to win the presidency. The allotment of electors for each state is the number of members of its congressional delegation: two for each senators and one for each Representative in the House (for example, New York State has 29 electors as there are 27 Representatives and two Senators).
The District of Columbia, which has no representation in Congress, is awarded three electors, the same number as the state with the least electoral votes.
The selection of electors is actually a two-step process. First each candidates’ party selects its slate of electors for each state. These can be state elected officials or party leaders, and are usually selected for their loyalty to the party.
Electors cast their votes, one for president the other for vice-president, in December and those votes are then counted in Congress in early January. If an elector votes for a candidate other than the one they are pledged to, they are known as “faithless electors”. In the the history of US elections there have been only a handful and have never decided an election.
There have been five elections where a candidate has won the Electoral College vote but lost the popular vote. The most recent was the latest in 2016 when Donald Trump lost the popular vote to Hillary Clinton but won the election.
Throughout the years, particularly after very close elections or when a candidate wins the presidency without winning the popular vote, there have been calls to change the Electoral College system and some states have proposed measures to switch to a proportional system, but none have been enacted.