Around this time in an election cycle, candidates always begin trying to win over the voters who make up the largest share of the electorate, and who usually vote for the winner. We're talking, of course, about women.

In the mid 1990s, there were the soccer moms, the suburban swing voters who everyone assumed were a presidential election's secret weapon for over a decade. In October and November 1996, there were nearly 200 articles mentioning soccer moms in major newspapers. A Boston Globe columnist called 1996, 'the Year of the Soccer Mom.'' In the 2004 presidential election, there were the security moms and malpractice moms. In 2008, there were hockey moms. In 2012, there was the war on women -- one that's still percolating as a rallying cry through both parties today. As the "war on women" rhetoric -- originally focused on reproductive rights -- shows, the exact slice of female voters that politicians think are crucial to their success has  changed over time, but it's still a campaign story that hits reprise every election cycle.

This chart from PBS Newshour helps explain why. 

Source: PBS Newshour

Since 1976, women have reliably voted slightly more for Democrats than they have for Republicans. They have also made up a majority of the electorate since 1984. If Democrats didn't hold on to their advantage with women, it could be bad for their future electoral chances. Which is why Democrats have increasingly focused on minority and unmarried women, who haven't turned out in past elections but could be a formidable addition to the Democratic coalition.

Source: Voter Participation Center
Source: Voter Participation Center

In midterms, even fewer unmarried women vote. In 2008, 20.3 million unmarried women voted in the presidential election, according to the Voter Participation Center. In the 2010 midterm, 10.1 million fewer women turned out, a 33 percent drop. Married women, on the other hand, are voting more Republican lately. By focusing on unmarried women, Democrats are targeting only the voters already inclined to support their platform, if not enough to go vote.

In midterm elections, where the frame has already been set to depict Democrats fighting an uphill battle, they can't win without these voters. Which makes the Obama administration's recent push on equal pay make all the more sense. Women make up two-thirds of all minimum wage workers, and many of the unmarried and minority women who haven't figured heavily in the electoral calculus in previous cycles are the ones who could be most affected by equal pay legislation and Obama's two executive orders on pay transparency.

On Tuesday, President Obama framed the equal pay legislation with the same "war on women" rhetoric that the Democratic Party has used since 2012. “This isn’t just about treating women fairly. This is about Republicans seemingly opposing any efforts to even the playing field for working families. I don’t know why you would resist the idea that women should be paid the same as men and then deny that that’s not always happening out there.”

The Senate failed to pass the Paycheck Fairness Act this morning. No Republicans voted for it.

Republicans are trying to make inroads with women in other ways, either to hold on to their slim advantage with married women -- already more likely to turn out -- or try to win over unmarried women. Republican donors held a big fundraiser for three women running for the House last month. The Republican National Committee held an event for their "rising stars" at a winter meeting, which featured several female candidates.

The communications director for Concerned Women for America said, "The war on women is directly the left's attempt to narrow women down to one-issue voters, and that's not true. But the war for women is an attempt for women's vote. Because it is a large demographic, a huge voting bloc, and we should be honored so much that people are trying to figure out enough what we care about to speak to us." The RightNOW Women's PAC launched this year, which Sen. Kelly Ayotte (R-N.H.) called, “Democrats’ worst nightmare.”

The future of parties trying to woo women might fall more along these lines -- Democrats winning younger and minority women, and Republicans winning older, married women.