Harvard professor Larry Lessig, briefly a Democratic candidate for president last year, says a growing number of electors are considering a plan to thwart Donald Trump.

The constitutional law professor has been offering pro-bono legal advice to Electoral College members through his anti-Trump group Electors Trust.

He told Politico the group has heard from 20 Republicans who are open to defecting from Trump and becoming 'faithless electors.'

That's 'more than half' the number necessary to keep Trump from becoming president, he said.

Scroll down for video

Harvard professor Larry Lessig, briefly a Democratic candidate for president last year, says a growing number of electors are considering a plan to thwart Donald Trump

Trump is slated to win 306 electoral votes. Should 37 electors defect, he'll fall below the constitutionally-mandated threshold of 270, and the House of Representatives will get to select the next president.

Faithless: Republican never-Trump activist Chris Suprun (right) is the only known Republican intending to become a faithless elector

The Republican National Committee has only identified one party member in its whip count willing to defy the Trump, the presumptive president-elect.

Texas elector Chris Suprun signed an open letter to Director of National Intelligence James Clapper asking for an intelligence briefing before the Dec. 19 Electoral College vote.

Suprun and a group of nine other electors that includes the top-ranking House Democrat's daughter, Christine Pelosi, demanded a security briefing on government claims that Russia interfered in the election before they cast their votes.

Hillary Clinton's campaign backed them up on Monday, saying, 'The bipartisan electors' letter raises very grave issues involving our national security.

'Electors have a solemn responsibility under the Constitution and we support their efforts to have their questions addressed,' campaign chairman John Podesta said.

A separate group of electors whose votes are bound to Hillary Clinton are challenging laws requiring them to vote for the candidate who won their state.

They're brokering deals with Republicans who don't want Trump in the White House, either.

If they're able to throw enough votes, the Republican-run House will take its turn. Lawmakers would have the opportunity to put someone else in the Oval Office instead of Trump.

Delegates from a majority of states are required to mark down the candidate who won the most votes in their statewide election. But 20 states have unshackled votes.

Trump is slated to win 306 electoral votes. Should 37 electors defect, he'll fall below the constitutionally-mandated threshold of 270, and the House of Representatives will select the next president

'Obviously, whether an elector ultimately votes his or her conscience will depend in part upon whether there are enough doing the same,' Lessig told Politico. 'We now believe there are more than half the number needed to change the result seriously considering making that vote.'

Politico says that Lessig did not provide evidence of his claim, which counters public and private whip counts.

The Electoral College has never defied the will of voters. In two cases candidates did not command a majority, and the decision went to the House of Representatives - in 1800 and 1824.