In a rare expression of protest, a Georgia Republican says he might withhold his vote for Donald Trump from the Electoral College after November's election.

In an interview with the the Atlanta Journal-Constitution, Baoky Vu, a Vietnamese immigrant who is one of 16 GOP electors in Georgia, cited Trump's current feud with the parents of a Muslim U.S. soldier killed in Iraq. He said that he won't vote for Trump in November and also that "I have the right to vote for a write-in candidate in the Electoral College."

But does he? And what is this all about? Below, we explain.

How rare is it?

First, it bears noting that Vu's comments are striking both because this doesn't often happen in American elections and also because the elector is promising to withhold a vote from Trump before the general election is even held — apparently trying to send a message. Yes, Trump will probably carry red-leaning Georgia and its 16 electoral votes — and if he doesn't, the election probably won't be close enough for a single electoral vote to matter. But Vu's decision to suggest beforehand that he might be a so-called "faithless elector" is certainly unusual.

As for faithless electors writ large, they're exceedingly rare.

According to the voting reform group FairVote, there have been 157 faithless electors in American history. About 45 percent of those electoral votes were changed because the candidate died before the Electoral College voters were tallied. Among the rest, three electors chose to abstain, while 82 voted for a candidate other than the one they were required to. That's fewer than two true faithless electors per election, given that we've had 57 presidential elections.

The last time we had a faithless elector in a presidential campaign was 2004, when an anonymous Minnesota elector voted for John Kerry's running mate, then-Sen. John Edwards (D-N.C.), rather than Kerry. It was widely thought to be an error rather than a protest vote.

Prior to that, D.C. elector Barbara Lett-Simmons in 2000 abstained, citing the District's lack of voting representation in Congress. In 1988, a West Virginia Democratic elector did what the Minnesota elector did and cast a ballot for vice-presidential candidate Lloyd Bentsen rather than Michael Dukakis for president. In 1976, an impatient Washington state Republican voted for Gerald Ford's primary opponent, Ronald Reagan, instead of Ford. Richard Nixon lost one elector each in the presidential elections of 1960, 1968 and 1972; all of them went not for the Democratic nominees but for other options.

Is this even legal?

Vu is correct that he does have that right — in Georgia, at least — to not vote for the person who wins his state's electoral votes. In other states, that's not the case -- but the penalties are usually small, including fines, and aren't enforced.

Although the Constitution does spell out the details of the Electoral College, it does not weigh in on how electors are supposed to vote. According to the National Archives, the Supreme Court has said that political parties may require electors to take pledges to vote for a particular candidate, but it has not weighed in on whether penalties for breaking that pledge are constitutional.

Georgia is one of 21 states without a faithless elector law. The other 29 and the District of Columbia do have such laws, but no faithless electors have ever been prosecuted, according to the Archives.

Part of the reason nobody has been prosecuted is because faithless electors have never changed the outcome of a presidential election. Which brings us to...

Could it actually hurt Trump in 2016?

Vu's threat to be a faithless elector will undoubtedly be seized upon by Donald Trump, who in recent days has taken to alleging that the 2016 election might be rigged against him. What better way to prove that than by pointing to Vu and the idea that other electors may change the result of the election even if Trump wins?

But as I noted, that has never happened.

There have been cases in which groups of faithless electors joined forces, but it has been more than a century. In 1986 and 1912, groups and four and eight electors, respectively, voted for vice-presidential candidates other than the ones they were pledged for. In 1872, 63 Democratic electors declined to vote for Democratic nominee Horace Greeley, who died after Election Day.

In fact, the only time that faithless electors could have changed the outcome of a presidential election was for vice president, and it was a long time ago.

In 1836, 23 Democratic electors from Virginia declined to vote for Richard Mentor Johnson for vice president because of his relationship with a black woman, leaving him shy of a majority needed to become vice president. But the Senate, which is tasked with resolving vice presidential elections in which no candidate gets a majority of the electoral votes, made Johnson vice president anyway.

Why it won't happen

As I noted, Vu's threat is probably counterproductive in that it gives Trump a talking point for his rigged-election allegation. But it has been 180 years since a conspiracy to stop a presidential or vice-presidential candidate from taking their rightful victory in the Electoral College has truly been hatched.

But there's very good reason for that, and it's the same reason Republicans didn't really try to stop Donald Trump from taking the GOP nomination at their convention two weeks ago: It would be overturning the will of the voters.

There's also the fact that faithless electors almost never cross over. Almost all of the faithless electors described above voted for either another member of their own party or a third-party option — not the other party's presidential or vice presidential nominee. So if a candidate — whether Trump or Clinton — lost the Electoral College majority due to faithless electors, it's not a given that it would automatically install their opponent. It's more likely the House would decide (the Senate gets to resolve undecided VP races, but the House gets to pick a president if nobody gets a majority). And indeed, Vu is threatening not to vote for Clinton but for a write-in candidate.

There certainly have been situations in which faithless electors could have changed the outcome of the election; given George W. Bush's five-electoral vote win in 2000, only two electors would have had to change their votes to throw the race to the Senate, and three crossovers (which again, are very rare) could have made Al Gore the president.

But the likelihood that the 2016 election would be close enough for faithless electors to make a difference and that there would be enough of them to actually change that outcome makes it a very remote possibility.

Of course, that won't stop Trump from talking about it. And we will have Vu to thank for that.