Democratic hopes of holding the Senate don’t just turn on the ability to win big among female voters, an imperative that discussions of the “gender gap” and the GOP’s “woman problem” often focus on. As I’ve reported here, those Dem hopes will also require a successful push to make the composition of the electorate more female, too.

The new Washington Post/ABC News poll underscores still another layer to the challenge Democrats face: It finds that Republican women are certain to vote this fall at a significantly higher rate than Democratic women are.

Broadly, the poll finds that the familiar Democratic “midterm drop-off problem” is very much in force. It finds that 63 percent of Americans are “absolutely certain to vote,” and these numbers are higher among GOP-aligned groups (whites, older voters) than among Dem ones (non-whites, younger voters):

* 77 percent of Republicans are certain to vote, while 63 percent of Democrats say the same.

* Only 39 percent of voters 18-39 are certain to vote, while 73 percent of voters over 65 say the same.

* Only 45 percent of nonwhite voters are certain to vote, while 70 percent of white voters say the same. (Philip Bump has a terrific interactive chart that shows all of this in more detail.)

Move to women, and it gets very interesting. Sixty-five percent of women say they are certain to vote, versus 61 percent of men. That’s a plus for Democrats, right?

Well, you’d think, but when you break down those women by party, it turns out that 79 percent of Republican women are certain to vote, versus 66 percent of Democratic women.

This gets to the core of the Dem challenge this year: Turning out Democratic women. Our poll didn’t break this down by marital status, but given that unmarried women lean Democratic and tend to be less likely to vote in midterms, it seems reasonable to assume marital status may be one thing that’s driving this gap. And so, while Dem operatives are determined to push the female share of the vote in key races up to 53 percent or higher, their ultimate success requires turning out the right women.

The Dems’ vaunted Bannock Street Project is all about this. “Getting out the low propensity women is absolutely critical for Democratic chances of holding the Senate,” political scientist Michael McDonald, who is tracking early voting, tells me.

McDonald says one way to track whether this is working is to look at early voting numbers, which the Bannock project is already working to boost, through absentee ballots. His current tally shows that Democrats are leading among absentee ballots requested in North Carolina and Iowa. But he says that Republicans are outperforming their absentee ballot totals.

The key question then becomes: Are Republicans merely getting people to vote early who would have voted anyway? The DSCC insists that this is what is happening, claiming that Democrats are bringing in absentee ballots from voters who didn’t vote in 2010. This is a key metric to keep an eye on, and if Democrats are in fact doing this, it should show up soon in state polls, since it should theoretically help widen the “likely voter” electorate in those polls. Right now, it looks pretty bad for Democrats: The state polling averages show Republicans with a modest to significant edge in all of the key races.

“If the Bannock Street Project is doing what it claims it is doing, which is convincing moderate propensity voters to vote, then once the early voting really piles up, the polls should shift in a Democratic direction,” McDonald tells me. However, he cautions against reading too much into national polls to judge this: “If Bannock is being successful, it will be difficult to see that in the national numbers, since they are focused on specific states.”

So the national numbers I’ve highlighted above may not tell us too much about what’s going to happen. But they hint at the general nature of the challenge Democrats face, and the degree to which it turns on the relative lack of enthusiasm for voting among Dem-leaning women. Yep, it’s all about the women.