President Obama speaks to students and faculty from the Kellogg School of Management at Northwestern University on Oct. 2 in Evanston, Ill. Obama highlighted the nation's recent economic improvements during his speech. (Photo by Scott Olson/Getty Images)

On Nov. 5, if the election turns out the way almost every forecast suggests that it will with Republicans taking the Senate, Democrats will likely blame the failure of their voters -- young people, African Americans and Latinos -- to turn out. And given that many states with tough Senate races are in the South, where the majority of African Americans live, Democrats are very much banking on black voters to be their firewall.

A story from The New York Times blares "Black Vote Seen as Last Hope for Democrats to Hold the Senate."

And in the USA Today, "Black lawmakers anchor Democratic Southern Voter Push."

President Obama, who spurred black voter turnout to record numbers in 2008 and 2012, is making his own appeal to black voters -- an effort backed by a $1 million push on black radio in North Carolina and more in other states from the Democratic Senatorial Campaign Committee. (Of course, given the tens of millions the DSCC spends, this is a significant -- though not huge -- investment.)

Here was Obama on the Steve Harvey show on Wednesday morning (emphasis mine):

The truth of the matter is, African American voters, young voters, progressive voters, Latino voters, they now vote at relatively high rates during presidential elections. But I bet a whole bunch of your listeners aren’t even thinking about this election coming up on Nov. 4. But this is really the last election in which I have the opportunity to get a Congress that will work with me.

Back in 2010, folks didn’t vote. As a consequence, the tea party took over the Republican Party. We lost the House. And, although we’ve made a lot of progress on various issues since then, basically Congress has fought me every step of the way and it led to things like the shutdown and all kinds of negative consequences in terms of things like gun control that we couldn't get done. So, we really need to have the kind of Congress that is serious about the issues that matter to folks and the responsibility is ultimately up on everybody’s who’s listening.

Folks like to complain, talk about Washington. But if only 45 ... 40 percent of the people are voting, then it’s not surprising that Congress isn’t responsive. If people voted at the same rates during midterms as they did during presidential elections, we would maintain Democratic control of the Senate . . . and so I need everybody listening to understand this is really, really important.

Harvey's audience is primarily black, spread across 64 markets and numbers 7 million listeners.

Obama said "folks didn't vote," and as a result he was handed a shellacking in 2010, courtesy of the tea party and the failure of his folks to show up at presidential rates. Keep in mind, though, that turnout drops across the board in midterms, often from around 60 percent to somewhere in the 40s, depending on the demographic.

Here's a breakdown of that midterm dropoff  going back to 1978:

In Obama's telling, the midterm drop-off, as reliable as almost anything in politics, necessarily spells doom for Democrats. And if Democrats can't crack the midterm code, then somehow they will come up short against Republicans.

Except it hasn't really worked that way.

"Many people are quick to lay Democratic losses at the feet of women, minorities, and younger people, but it’s simply not borne out by the data," said Margie Omero, a Democratic pollster. "Never mind comparing presidential turnout to midterm turnout. We can compare a Democratic midterm wave (2006) to a Republican midterm wave (2010). The demographics of those two electorates was nearly identical. Yet the results were vastly different."

The difference between 2006 -- a strong Democratic year -- and 2010 had more to do with how the electorate that did vote felt about the party and the president. Among whites, the generic ballot vote (i.e. would you vote for a generic Republican or a generic Democrat) was 60-37 in 2010. In 2006, though, the generic ballot among whites was much more evenly distributed among the parties, 51-47 for Republicans.

And in fact, black turnout in 2010 was actually up from 2006 and the highest in a midterm since 1986. And still, Democrats lost big.

So what else could account for the Democrats' midterm woes? How about their declining fortunes among Southern whites.

Georgia, for example, has such strong black population growth that it gained an electoral vote after the most recent Census. But at the same time, white voters seem to have coalesced even more behind the Republican Party. Only 16 percent of the state's white population backed Obama in 2012, by some estimates.

Obama got a similar level of white support in Louisiana, which also has a sizeable black population. Other key 2014 Senate states like Arkansas and Kentucky have smaller black populations, but also high levels of support among whites for the Republican Party. That gap among white voters is a much bigger factor than black turnout for Democrats. Even if, as Jonathan Capehart has urged, the Obama coalition proves everyone wrong by voting in strong, presidential-election-like numbers, it would be unlikely to offset the strength of the anti-Democrat white vote.

Black turnout, you'll see in the chart above, has dropped by 15, 19 and 21 points in each of the last three midterms, from the preceding general election, with that margin growing each year. White turnout, meanwhile, dropped 13, 15 and 17 points in those same elections -- so a slightly smaller dropoff.

But that dropoff among black voters -- even if it's just a couple points more than white voters -- is still significant, especially since more than 90 percent of African Americans vote Democratic. That means for every 1 percent of black voters who stay home, that's basically one percentage point coming at Democrats' expense.

This is why you're seeing such a strong push for black votes. They are undoubtedly a vital piece of the Democratic coalition.

But so are whites. And come November, it's more likely Democrats will lose the Senate because of Southern whites than because of Southern blacks.