At least 20 GOP Electoral College voters are considering voting against President-elect Donald Trump next week, according to a Harvard constitutional law professor who’s offering legal aid to “faithless electors” who betray their party.

Harvard University Professor of Law and Leadership Larry Lessig did not offer evidence to support his claim. But, if true, the figure would put the anti-Trump movement’s goal of scuttling his election win much closer to becoming reality when 538 members of the Electoral College vote in their state capitals on Monday.

“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.”

On Monday, 10 members of the Electoral College — nine Democrats and one Republican — demanded an intelligence briefing on Russian interference in the presidential election. In a letter to the director of national intelligence, the electors said they wanted information on whether Trump and his associates have ties to Russia, saying the topic directly impacted the “core factors in our deliberations” as to whether Trump is fit for the White House.

Chris Suprun, an outspoken critic of Trump, was the only GOP elector to sign the letter.

Lessig, who briefly ran for president in 2016 as a Democrat, provides free and confidential legal support to electors wishing to exercise their “independent and nonpartisan judgment,” according to his website.

“We will defend you against any fines or legal claims that might threaten the freedom of your vote,” the website reads. “If you are an Elector, we will also allow you to know how many others like you there are. How many, not who. Because we will never reveal any Elector’s views, to anyone, ever.”

It’s unclear whether any of the electors considering voting against Trump live in states with laws that legally force them to vote for the candidate who won the state’s popular vote or be replaced by an alternate. It’s also unclear whether any Republicans in those states who did not vote for Trump would actually be counted, although similar laws are now being challenged in court, Politico reports.

Trump won 306 electors to gain the presidency last month; Hillary Clinton took home 232. If all of those Republican electors vote for Trump, he will easily surpass the threshold of 270 needed to win the White House. But if 37 Republicans decide to vote against the president-elect, that would leave Trump one vote short at 269.

Lessig told MSNBC that he’s confident there are at least 20 “faithless electors” right now — and the number could grow.

“There are a bunch of groups that are working to advise and support Republican electors who are interested in exercising a constitutional freedom to vote their conscience,” Lessig said. “That freedom comes from the federal Constitution … these are federal officials.”

Lessig said he believes at least 37 GOP electors will ultimately decide not to back Trump, creating a “very interesting” dynamic. The number could already be as high as 30, he told MSNBC.

“Our goal is to let the electors exercise their judgment, and what we believe is that at least 37 electors will make the judgment not to support Donald Trump,” he said. “And if that happens, then of course, it goes to the House and the House has to pick among the top three candidates.”

In 1808, six Democratic-Republican electors rejected James Madison, the most ever to reject a presidential nominee.