I continue my series on key demographics to watch in the very close election between President Barack Obama and former Governor Mitt Romney. My numbers are taken from an aggregation of polls, both online and by telephone, conducted by JZ Analytics between January and June of 2012. They represent the views of 4,500 voters nationwide.

Pollsters began to talk seriously about a gender gap in national elections in 1988 and, at times, the differences in voting and issues emphasis between men and women has often been striking. What doesn't receive enough attention, however, is the even more dramatic "marriage gap" - how married voters and single voters see their world so differently. In an analysis provide by my colleagues at Gallup after the 2008 election, all married voters supported Senator John McCain over his Senate counterpart Barack Obama by a 12 point margin - 56% to 44%. Single voters favored Obama overwhelmingly by 30 points - 65% to 35%. But the gender characteristics of voter patterns were nearly canceled out by the marriage gap. In 2008, married men gave McCain a 16 point advantage over Obama - 58% to 42% -- while single men pounded the Arizona Senator by 26 points - 63% to 37%. Married women supported McCain by 6 points, 53% to 47%, but it was Obama's 32 point margin among single women - 66% to 34% -- that provided him with a comfortable margin of victory.

We see every indication thus far that married voters will turn out and we see pretty much the same patterns as in 2008 in our 2012 polling. Thus, Romney leads Obama among married voters by 13 points - 50% to 37% -- which is about the same as McCain's margin in 2008. But Obama's 18 point lead among all single voters - 52% to 34%-- is far short of his performance four years ago. Again the Republican leads among married men by 18 points - 51% to 33% -- even better than McCain's margin. Obama's margin in our poll of 27 points (56% to 29%) among single men is also on the mark with 2008. Romney leads among married women this time by 10 points (50% to 40%), better than McCain's 6 point margin. But, while Obama leads among single women by 21 points - 50% to 29% -- this does not match his 32 point margin in 2008. Even worse for Obama, at this point in time, is that 21% of single women are undecided. This is a strong suggestion that they may not even vote.

This is the "marriage gap" and right now it is pretty clear why Obama is not performing well.