Two members of the electoral college in a US state where Hillary Clinton won the popular vote have launched a last-ditch lawsuit to block Donald Trump from winning the American presidency.

The two electors in Colorado, Polly Baca and Robert Nemanich, have legally challenged a state law that requires them to vote for the winner of the popular vote, in the hope they can instead rally around an alternative Republican candidate in an effort to oust Mr Trump.

While Colorado’s official vote is yet to be finalised, unofficial results show Ms Clinton won 48.1 percent of the vote, compared to 43.25 percent for Trump, meaning the state’s nine electoral voters are compelled to vote for her when the electoral college meets on 19 December.

But speaking to the Denver Post, Jason Wesoky, the attorney who filed the suit, said the electors wanted to “vote their conscience and do their constitutional duty as intended by the framers”.

They are among around seven so-called faithless electors - who also call themselves the “Hamilton Electors” - hoping to persuade Republican electors in other states to vote for a moderate Republican candidate by offering to shift the Democratic votes to a consensus pick.

So far, one Republican elector has declared he will not vote for Mr Trump. On Tuesday, Christopher Suprun, a Republican member of the electoral college in Texas, announced he would not cast their vote for the billionaire, saying he “owes a debt to [his] children” not to select an unqualified President.

At the time of writing, Ms Clinton had 2.7 million more votes than the President-elect in the popular count, but lost the election in November because of the way the United States' Electoral College system works — a result that academics have suggested could be ruled unconstitutional.

In the US, rather than citizens voting for the head of state directly, representatives in the Electoral College choose the winner on behalf of their state, and almost all states operate a “winner-takes-all” system, which ignores voter margins, meaning the candidate with the wider percentage will receive every elector's vote.

A candidate needs 270 votes form the Electoral College to win and become president. Mr Trump has 306 projected votes to Mrs Clinton’s 232, meaning there are 306 Republican electors and 232 Democratic electors. The Hamilton Electors would therefore need all 232 Democrats to go along with a Trump alternative as well as 38 Republicans.

Polly Baca told the Colorado Independent: “I think we all agree that Mr Trump is a danger to this country. I have been involved in every presidential race since 1960 and this is the first time that I am literally in fear for my country because of this particular person.”

The lawsuit states: “Though Hillary Clinton and Timothy Kaine won the majority vote in Colorado and are qualified for office, plaintiffs cannot be constitutionally compelled to vote for them.

“Plaintiffs are entitled to exercise their judgment and free will to vote for whomever they believe to be the most qualified and fit for the offices of president and vice president, whether those candidates are Democrats, Republicans or from a third-party.”

There is no federal law requiring electors to vote for the winner of the popular vote, but Colorado is among the 29 states that impose such a requirement. Republican Secretary of State Wayne Williams has reportedly pledged to replace the electors if they do so.

Mr Williams said Ms Baca and Mr Nemanich were "faithless electors" who were “seeking to conspire” and "violating Colorado law", saying in a statement: “Instead of honouring the will of the Coloradans who voted for them, these two faithless electors seek to conspire with electors from other states to elect a president who did not receive a single vote in November.

“Make no mistake, this is not some noble effort to fight some unjust or unconstitutional law; rather, this is an arrogant attempt by two faithless electors to elevate their personal desires over the entire will of the people of Colorado. And in so doing, they seek to violate Colorado law and their own pledges.

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Mr Williams went on to suggest that the move was "odious" and "evil", saying: “The very notion of two Colorado electors ignoring Colorado’s popular vote in an effort to sell their vote to electors in other states is odious to everything we hold dear about the right to vote.

"It is this type of evil that President Franklin Roosevelt warned us about when he cautioned that voters — not elected officials such as these faithless electors – are ‘the ultimate rulers of our democracy'.”

One potential candidate as an alternative to Mr Trump is Ohio Governor John Kasich, but he issued a statement Tuesday telling electors not to vote for him.

Despite the growing size of the rebellion, few entertain the belief that faithless electors alone can overturn Mr Trump’s 306 to 232 victory in the electoral college system.

And some analysts predict that even if the electoral college managed to select Ms Clinton, Congress would go ahead and appoint Mr Trump anyway.