Donald Trump has used his latest overnight Twitter tirade to rail against proposals among some Democrats to abolish the Electoral College system and lower the voting age to 16.

“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.

Pete Buttigieg and Beto O’Rourke, two other leading Democratic candidates, have also expressed similar approval for voting reform.

Who could be running against Trump in 2020? Show all 23 1 /23 Who could be running against Trump in 2020? Who could be running against Trump in 2020? Joe Biden The former vice president - poised to be a frontrunner - has announced his run. He recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well AFP/Getty Who could be running against Trump in 2020? Bernie Sanders The 2016 runner-up has announced that he will be running again in 2020 Getty Who could be running against Trump in 2020? Hillary Clinton The 2016 Democratic presidential candidate and former Secretary of State says she is “still considering” whether she will run again. Getty Who could be running against Trump in 2020? Pete Buttigieg The Indiana mayor and war veteran will be running for president. If elected, he would be the first openly LGBT+ president in American history. Getty Who could be running against Trump in 2020? Kamala Harris The former California attorney general will be running for president in 2020. Introduced to the national stage during Jeff Sessions’ testimony, she has endorsed Medicare-for-all and proposed a major tax-credit for the middle class. AP Who could be running against Trump in 2020? Elizabeth Warren The Massachusetts Senator has formally launched her bid for president in 2020. A progressive Democrat, she is a major supporter of regulating Wall Street. AP Who could be running against Trump in 2020? Beto O’Rourke The former Texas congressman told Oprah Winfrey that he “has been thinking about running for presidency”, but stopped short of formally announcing his bid to run in 2020. AFP/Getty Who could be running against Trump in 2020? Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam has announced his bid. He intends to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord. Vice News Who could be running against Trump in 2020? Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege.” Getty Who could be running against Trump in 2020? Cory Booker The New Jersey Senator has announced that he will be running for the presidency in 2020. If he secures the nomination he said finding a female vice president would be a priority. Getty Who could be running against Trump in 2020? John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017. AP Who could be running against Trump in 2020? Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US. Getty Who could be running against Trump in 2020? Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but is likely to face tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad. Getty Who could be running against Trump in 2020? Andrew Yang The entrepreneur has announced his presidential candidacy, and has pledged that he would introduce a universal basic income of $1,000 a month to every American over the age of 18. AFP/Getty Who could be running against Trump in 2020? Marianne Williamson The author and spiritual advisor has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful. Getty Who could be running against Trump in 2020? John Kerry The former secretary of state has said he is still thinking about whether to run. Getty Who could be running against Trump in 2020? Michael Bloomberg The entrepreneur and former New York mayor– with a net worth of around $50bn – has said he will decide by the end of February whether to seek the presidency. AFP Who could be running against Trump in 2020? Howard Schultz Former Starbucks CEO Howard Schultz has not yet ruled out running for president in 2020, despite criticism that his bid could help re-elect Mr Trump by dividing the Democrat vote. AP Who could be running against Trump in 2020? Eric Holder The former attorney general has said he will decide in “the next month or so” whether to run as a 2020 presidential candidate. AP Who could be running against Trump in 2020? Eric Swalwell The California congressman said he is “ready to do this” and will decide before April whether to run. MSNBC Who could be running against Trump in 2020? Terry McAuliffe The former Virginia governor, who worked to elect Democratic governors during 2018 midterms, said there was a “50 per cent” chance he would run. AP Who could be running against Trump in 2020? Sherrod Brown The Ohio senator is still undecided about whether to run for president in 2020. Who could be running against Trump in 2020? Mitch Landrieu The former New Orleans mayor said he doesn’t think he will run for president, but “never say never”. AP Photo/Jacquelyn Martin

"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.

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.

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”.

Presidential candidate Kamala Harris vows to 'prosecute the case' against Donald Trump

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.