Rep. Lucille Roybal-Allard's (D-Calif.) reelection campaign in California's 40th district will be decided by fewer than 300,000 eligible voters.

By contrast, the new congressman in Montana's lone congressional district has to appeal to an electorate of 778,000 potential voters.

How is this possible in a country where equal representation is written into the Constitution? Well, it's pretty simple: All eligible voters are people, but not all people are eligible voters. And some districts have huge numbers of people who are not able to cast ballots.

In fact, seven of the United States's 435 congressional districts have more people who aren't eligible to vote than people who are, according to data from the Pew Research Center.

Tops among them is Roybal-Allard's district, in which 57 percent of her constituents are not eligible to vote. This, of course, could be because they are too young, not citizens, or both. And given Roybal-Allard's district is 87 percent Latino -- the highest in the nation -- it's no surprise that she's got the fewest eligible voters.

Montana is on the other end of the spectrum. It's only about 3 percent Hispanic, and its lone House district has more people than any other in the country. That's because, as a one-district state, it's on the verge of being split into two districts -- though not quite there (just a few more years till 2020, Montana!). And because districts must be apportioned within state boundaries, many states with just one district have oversized or undersized congressional districts.

When it comes to statewide or national elections, districts with significantly fewer eligible voters have considerably less sway than districts with lots of eligible voters. Yet each of them elects one representative to the U.S. House.

Here's a map of all 435 districts, according to what percentage of their population is eligible to vote:

You'll notice that looks a lot like this map of Latino population by district:

Want to peruse all the glorious data yourself? Have at it.