Washington(CNN) Presidential elections are a state-by-state battle. But in two instances this November, it could be district by district.
Maine and Nebraska are the only two states in the nation that don't have a winner-take-all system for their Electoral College votes. That means that even if a candidate doesn't win the statewide popular vote in those states, they could pick up one or two Electoral College votes if they win in congressional districts.
And with an increasingly close election, both Republicans and Democrats see an opportunity to add purple to the map.
Brief civics lesson: You're not technically voting for president on Nov. 8; you're voting for electors. It's those people -- 538 of them -- who technically pick the president. There's a math equation used to calculate how many electors each state gets: three Electoral College votes based on the total number of US senators and US representatives it has (and three extras for the District of Columbia).
While individual electors may technically cast those votes for whomever they choose, it is virtually unheard of for a candidate who wins the popular vote in a state to not get all the electoral votes, and many states have laws enforcing that electors must respect the statewide vote.
Except, that is, in Maine and Nebraska, where things can get complicated. In those states, the votes are tallied by congressional district.
In Maine, which has two senators, two congressmen and so four electoral votes, the statewide winner automatically gets two votes. And then the winner of each of the two congressional districts gets one electoral vote, respectively.
In Nebraska, there are three congressional districts, meaning two votes go to a statewide winner and the three districts each award one electoral vote.
So even if a candidate wins the majority of either state, Maine could have a 3-1 split and Nebraska could have a 4-1 or 3-2 split.
This has almost never mattered -- but this year it might. If things get really close.
They're not that close right now. Hillary Clinton has a strong advantage in the Electoral College. But CNN rates one congressional district each in normally red Nebraska and normally blue Maine as a "battleground."
Maine has had the proportional system in place since 1972 and Nebraska since 1991, and only once have the votes ever split.
Barack Obama in 2008 got one electoral vote from deep red Nebraska from the Omaha area. The President reflected on the unprecedented feat from 2008 while visiting Omaha in January.
"That year, in the primary, I won the Nebraska caucus, and there some people, I saw the signs calling the city 'Obamaha,' and then in November of 2008, Joe Biden and I won the electoral vote," Obama said during a trip to Nebraska. "And then four years later I got whooped, all across this state."
Republicans in Nebraska have tried to change the law but failed by one vote this April to switch back to a winner-take-all.
Both sides have their eyes on those single, accessible electoral votes.
Trump has paid attention to and campaigned in Maine, a traditionally blue state in presidential years. He has reserved $47,000 in campaign ad time there from later this month through Election Day.
Clinton, on the other hand, has paid attention to Nebraska, a traditionally red state in presidential years. She held an event with billionaire investor Warren Buffett in Omaha in early August, and has spent more than $500,000 on ads in the Omaha market. She has more than $120,000 reserved there through Election Day.
Polls have tightened since early August, when Clinton had an advantage in many swing states that could have made for a landslide victory. Since then, races in many states have been toss-ups, making every electoral vote important.