A number of Democrats have reportedly agreed to break with tradition and vote against Donald Trump when the Electoral College meets on December 19 in an unlikely attempt to block his presidency.

At least half a dozen Democratic electors are preparing to lobby voters in other states in an attempt to keep the Republican out of the White House but, more significantly, also to undermine the legitimacy of the institution.

The voters, who are mostly former Bernie Sanders supporters from Washington state and Colorado, are pushing for other electors to ignore their oaths by voting against Mr Trump.

It is extremely rare for an elector to disobey the will of the public, and in certain states voters are bound by state law to follow the popular vote.

However, even if a number of “faithless electors” vote against convention, they're unlikely to persuade the necessary 37 Republican electors to reject Mr Trump.

At last count, Mr Trump had won 290 electoral votes, while Hillary Clinton had gained 232. The results in Michigan, which has 16 electoral votes, remain too close to call. A 270-vote majority is needed for a candidate to win the presidency.

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However the Democratic electors are pushing to undermine the entire Electoral College system and they hope their efforts could bring about major changes to the 228-year run constitutional process.

"I do think that a byproduct would be a serious look into Electoral College reform," Democratic elector Micheal Baca, from Colorado, told Politico.

"If it gets into the House, the controversy and the uncertainty that would immediately blow up into a political firestorm in the U.S. would cause enough people — my hope is — to look at the whole concept of the Electoral College," said another elector involved in the anti-Trump planning.

After a bloody campaign wounding both Democrat and Republican camps, the group is reportedly considering urging electors to also oppose Ms Clinton, and partner with Republicans in support of a new candidate like Mitt Romney or John Kasich.

"If you could get eight or 10 Trump electors to vote for someone else then that would probably get people's attention," said George Edwards III, a political science professor and Electoral College expert at Texas A&M University.

"We haven't ever had that many faithless electors in one election."

The Electoral College is made up of 538 members and the number of electors is determined by the number of senators and representatives for each state.

If the defectors go ahead with their plan on 19 December, it will be the most “faithless electors” since 1808, when six declined to vote for James Madison.