Black voters hold an unusual position in American politics. They vote heavily Democratic, as you know, and have for decades. The black population isn't as large as the white population, so that Democratic dominance doesn't tip the scales as much as it might otherwise; in general, the two parties are pretty evenly balanced when it comes to the national vote.

For Republicans with an eye on 2016, though, peeling off enough African American votes to shift the scales is a critical goal. The Post explored how this is working for the party's presidential candidates, including stops at the National Urban League conference on Friday. It makes sense. The black vote has become important enough to the Democratic party that a small drop in support could make a big difference.

If you look at House exit polling since 1992, you can see the long-term pattern of Democratic support from black voters.

Notice the size of the dark blue bars in 1992 and 2014 -- especially when compared to the size of the light blue bars.

Or, to break that out more explicitly, here's how the percentage of Democratic support in exit polling has changed -- and how much of that support has come from black voters.

The trend has been generally upward from the early 1990s to now -- particularly from 2008 on. This is analysis of exit polling, mind you, so there's some margin of error. But in the 1992 election, 13 percent of the Democratic vote came from black voters. In 2014, it was 23 percent.

That's the overall trend. On a state-by-state basis -- which is much more important in presidential politics -- it's more complex.

If we compare the total Democratic vote with the black Democratic vote in swing states with large black populations, you can see that the black vote increased in 2008 and 2012 in Florida and Ohio -- as it did nationally. In 2008, President Obama won the four swing states pictured. In 2012, he only lost North Carolina.

For the most part, shifting the black vote a small amount in those states wouldn't have made a difference in the outcome. For Obama to have lost Florida in 2008, he'd have had to see the black vote swing 11 points to the Republicans. To lose Ohio, it would have needed to swing 21 points.

Other states, though, would have had a different result with a far less dramatic swing. If there'd been a one-point swing from Obama to McCain in North Carolina in 2008, McCain would have won the state. And if the black vote in Florida and Ohio in 2012 had been as strong for the Republicans as it was in 2004 -- in the pre-Obama era -- Romney would have won both.

Still not enough to win the presidency by itself, of course! But a big improvement for the party. And that, in a nutshell, is why Republican candidates are happy to pick up a few percentage points where they can.

That does not mean that they'll be able to.