Black voters played a huge role in delivering Barack Obama to the White House in 2008 and 2012. And in 2014, they will play a huge role in determining whether the president's party can stop Republicans from taking the Senate.



Black voters in Virginia wait for President Obama to speak at a 2012 rally. (Marvin Joseph/Washington Post)

Why? Here are four reasons:

1. Black voters are hugely influential on the 2014 map

Six of the 16 states with the highest black populations are holding key Senate contests in 2014. A seventh -- the most African American state in the country, Mississippi -- is holding a contest that could get interesting if there's a tea party upset in the GOP primary.

This is a highly unusual set of circumstances, especially when you consider that most states with large numbers of African American voters generally don't hold competitive Senate races because they are safely red (in the South, generally) or blue (in the Northeast).

What's more, black voters don't just matter to a lot of races; they also matter to the most important races.

Three of the states listed above -- Louisiana, North Carolina and Arkansas -- are widely considered to be the most pivotal when it comes to the GOP's hopes of winning the majority. These three races are expected to be the difference between a GOP majority and a Democratic majority -- at least the way things look right now.

2. Black voters are among the biggest midterm dropoff voters

An increase in African American turnout was huge for President Obama's 2008 election and 2012 reelection, but history shows black voters are generally much less apt to vote than white voters, and that trend is even more pronounced in midterms.

The charts below, from the great Monkey Cage blog, show the states in which black turnout (as a percentage of registered black voters) has outpaced white turnout in recent elections. They are the green states. States where whites outpaced African Americans are denoted by increasing shades of red:

Looking at the last two presidential elections (on the right), a majority of states featured higher black turnout than white turnout in 2008, and most states with available exit poll data in 2012 showed the same.

In the 2006 and 2010 midterms (on the left), though, black turnout outpaced white turnout in just five and four states, respectively.

In addition, if you look just at the seven states with Senate races listed above, five of them had higher black turnout than white turnout in 2008, and three had the same in 2012. None of them had that distinction in either 2006 or 2010. And in many of them, black turnout lagged far behind white turnout.

3. Louisiana tells that tale

The above numbers come from the voter file (via Catalist). We've got more specific data from one of the most important states on the map: Louisiana.

The Pelican State just happens to keep detailed turnout numbers for all of its elections -- heroes! -- which allow us to make some more precise comparisons.

This chart looks at the percentage of the statewide vote that was black in every major election going back to the 1998 midterms (including odd-year governor's contests):

The average black share of the vote in the four presidential elections: 28.5 percent. The average black share of the vote in the other eight races: 26 percent.

Viewed another way, the average black share of the vote with Obama on the ballot is 30.1 percent. The average black share of the vote in the two midterms preceding Obama's wins drops to 25.3 percent. That five-point difference should not be underestimated, for the following reason...

4. Basically every black voter who stays home is a Democratic voter who stays home

PBS's Domenico Montenaro has noted that several key 2014 Senate states also have significant Latino populations. And while this important group is also prone to the midterm dropoff, it doesn't loom as large -- both because there are more black voters in these key states and because Latinos, while clearly favoring Democrats, aren't as politically homogeneous.

Black voters generally vote more than 90 percent Democratic, so just about every drop in turnout among black voters pretty clearly comes at Democrats' expense.

Let's say the black share of the vote drops 4 to 5 percent again in Louisiana in 2014. That's essentially 4 to 5 percentage points that a Democrat running in 2012 would have locked up but a Democrat running in 2014 -- Sen. Mary Landrieu (D), in this case, will have to make up.

And given that many of the states listed above have limited Democratic bases to begin with -- particularly in the Southern states where Republicans now dominate federal elections -- Democrats need to do something to get black voters to turn out in higher numbers than they have the last two midterms.

Expect Democrats to make significant pushes against the GOP's expansion of Voter ID -- which the left contends is targeted at reducing the black vote -- and the Supreme Court voiding part of the Voting Rights Act.

Their majority might very well depend upon it.