The Democrats are probably going to lose the Senate on Tuesday (or in a runoff in Louisiana or Georgia in the coming weeks).

What would make that even tougher to swallow for the blue team is that it will be in large part be because certain people simply didn't vote. With only a few exceptions over the last several decades, more Americans have identified as or leaned toward he Democratic Party than the GOP, according to data from Gallup. And Democrats even today often lead on the generic ballot among registered voters, but not likely voters.

But, for Democrats, it's not just the people who are registered to vote but choose not to; it's also people who choose not to even register to vote.

A new study from the Pew Research Center shows that, among people who either aren't registered to vote or are unlikely to vote in the 2014 midterm elections, 51 percent favor or lean toward the Democratic Party, while 30 percent favor or lean toward the GOP -- a 21-point gap.

Here's the demographic breakdown between non-voters and likely voters this year:

As you can see, non-whites, young adults and very low-income Americans are much bigger pieces of the non-voter pie than the likely voter pie. And all of these demographics were among President Obama's biggest supporters in 2012, voting for him by more than 20 points. The gap was bigger still among non-whites, who favored Obama by more than 40 points.

The chart above is why turnout and voter-registration efforts are such a big piece of what the Democratic party does. It knows its potential voters are out there, but they're also much less interested in showing up to cast a ballot. The GOP's base isn't so disinterested.

It's also why the Democratic Party is the one pushing for more early-voting hours, to get rid of Voter ID laws and to expand access to vote-by-mail options. They figure the more people voting, the more likely they are to win.

Tuesday's election, though, will not be a high-turnout affair.