Since the controversial contraception mandate exploded into a front-page story earlier this year, female voters’ voting preferences have become the subject of much debate. But, as the Pew Research Center’s new report, drawing on three decades of data, shows, the gender gap in presidential politics is not new. It does, however, have the potential to get significantly larger, as young, single women grow into an increasingly powerful voting bloc.

In Pew’s most recent national survey, conducted in early March, Obama led Mitt Romney by 20 points among women voters (58% to 38%). Considering that in both February and March, Obama ran about even with Romney among men, the gap among women is potentially perilous for Romney. But this commanding lead among female voters is nothing new – back in March 2008, both Democratic candidates, Obama and Hillary Clinton, were leading Republican rival John McCain by about 14 points among women voters. Obama ended up defeating McCain by 13 points among women (56% to 43%), while breaking even among men (49% to 48%).

Obama holds an even more imposing lead among younger women (age 18 to 49). In the March survey, Obama led Romney by 31 points among this group (64% to 33%). Among younger men (age 18 to 49), however, Obama and Romney are fairly even (50% vs. 46%).

So what’s behind the gender gap? Right off the bat, it’s important to note that a higher percentage of women identifies with or leans toward the Democratic Party. But, as Pew points out, there is also a marriage gap in party identification. In 2011, fully 62% of single women voters identified with or leaned toward the Democratic Party – while only 31% of single women voters identified with or leaned toward the GOP. There is no such gap among married women or single men, both of whom are more evenly split between the Democratic and Republican Parties. In contrast, married men tend to identify as Republican or lean towards the Republican Party.

With marriage rates falling and the women increasingly outnumbering men on college campuses, a new focus on young women as a coveted voting bloc could signal an important shift in the country’s politics. The U.S. Census has shown median age of first marriage for women rose from 21 (20.8) in 1970 to 26 in 2010. With more and more women remaining single into their late 20s and early 30s it is likely that the “single woman vote,” (or “swingle” vote, if you prefer) could become the demographic du jour.

In 2008, women significantly outnumbered men among Millennial voters (age 18 to 29), (55% to 45%). Obama beat McCain by 40 points among Millennial women (69% vs. 29%). It’s too early to predict how young women will vote in the 2012 election, but Republicans’ opposition to the contraceptive mandate could be one crucial rallying cry for Democrats seeking to get more young women to the polls. One thing’s for sure, though – both parties cannot afford to ignore this increasingly important demographic.