If Maine splits its electoral votes Nov. 8, which might happen, the results will be a rarity in American politics.

In the past 150 years, in only three elections have states deliberately divvied up their electors rather than handing all of them over to the presidential candidate who attracts the most votes.

It has never happened in Maine.

But in 2008, the only other state that has a system similar to Maine — Nebraska — awarded one of its four electoral votes to Democrat Barack Obama after the soon-to-be president racked up the largest tally in one of two congressional districts.

Maine and Nebraska give the winner of the statewide vote two electors and then allocate one to the candidate who comes out on top in each of the congressional districts. Maine adopted the system in 1972 and Nebraska in 1992.

Polls show Democrat Hillary Clinton is virtually certain to win Maine’s 1st Congressional District on Nov. 8 and is likely to capture the state as a whole. A new Emerson College poll Tuesday has Clinton up by 4 points in Maine.

But the 2nd District is up for grabs, with Republican Donald Trump having a real shot at capturing its electoral vote.

Spencer Kimball, an Emerson College professor and adviser to its polling society, said Tuesday the new poll found Clinton up 44-42 in the 2nd District with Libertarian Gary Johnson at 6 percent, Jill Stein of the Green Party at 2 percent and 6 percent of voters undecided. The poll had a 5 percent margin of error statewide, he said.

FiveThirtyEight, a well-respected polling site, gives Clinton a 53 percent chance of winning the district.

One electoral vote among 538 nationally obviously won’t make a difference in the outcome unless the presidential race is exceedingly close on Election Day. That Trump has visited Maine five times in the campaign says something about how seriously he’s taking the possibility that the state could provide a critical boost to his chances in a close race.

Whether or not Maine matters in the end, a split electoral vote would stand out in the annals of American politics.

Over the years, there have been occasional “faithless” electors who don’t wind up casting their Electoral College ballots for the candidates their states actually picked.

The last time that happened was in 1976 when an elector from Washington ignored Gerald Ford’s victory in his state and cast a ballot instead for Ronald Reagan, who had lost a primary to Ford. Many states now have laws barring electors from picking someone other than the winner from their state.

Other than the occasional faithless electors, in only a couple of races aside from 2008 has a state deliberately split its electoral votes: in 1960 and in 1892.

In 1960, Democrats in Alabama, divided on civil rights issues, had competing electoral slates. So even though John Kennedy won the state, he only got five of its electoral votes while Dixiecrat Harry Byrd walked off with six.

In 1892, Democrat Grover Cleveland won nationally against Republican Benjamin Harrison, but it got messy in the Electoral College, where Cleveland wound up snatching five of Michigan’s 14 electors even though Harrison won the state.

Stranger, though, was the outcome in North Dakota, which wound up giving one vote to Cleveland, one to Harrison and a third to populist James Weaver, who actually won the popular vote in the state.

In most presidential races, however, there are no state splits for any reason. Twenty-four of the 37 post-Civil War elections didn’t have any states with split delegations, not even a faithless delegate.

Until Nebraska’s split vote in 2008, the nation had seven straight presidential elections where each state’s electors cast their ballots for the same person. That matched the longest previous streak between 1920 and 1944.

Under the Constitution, it’s the Electoral College that actually elects a president unless it can’t agree, which would toss the decision to Congress.

Electoral votes are awarded to each state on the basis of how many senators and representatives the state has, ensuring that even the smallest states get at least three electoral votes.

The electors, typically chosen by presidential campaigns, don’t actually meet at any point. They just send in their ballots to the president of the U.S. Senate, who is normally the vice president.