THE slow decline of marriage is upending American politics. In the 2012 presidential election, unmarried women accounted for nearly a quarter of all votes cast. Their votes went decisively to Barack Obama, by 36 percentage points.

You might not think that a group that runs from not-yet-married college students to inner-city single mothers and divorced professionals had much in common. Yet unmarried women are spectacularly loyal to the Democrats—if they vote, which many do not. (Widows are outliers, voting more like married women.) The “marriage gap” dwarfs the sex gap, by which women as a whole have long favoured Democrats: Mr Obama beat Mitt Romney by a less dramatic 11 points among female voters.

Like boffins squabbling about quantum physics, some political types wonder whether “unmarried women” amount to a discrete voter block at all, or whether the label merely sweeps up various left-leaning slices of the electorate: ie, younger voters, poorer ones, more secular Americans and non-whites. That would be no more than an interesting metaphysical question, but for three big and inter-related developments.

First, unmarried women are one of America’s fastest-growing groups, leaping from 45m in 2000 to around 53m today—making them, in theory, a larger block of eligible voters than blacks and Hispanics put together (though in reality the groups overlap).

Second, Democrats—notably the Obama crowd—have found ways to map the electorate with unprecedented precision, using everything from polls and doorstep canvassing to commercial consumer databases. In the Dark Ages (ie, before 2008), campaigns might have blanketed majority-black city blocks or mostly-Democratic neighbourhoods with appeals to register and vote, while saturating the airwaves with paid advertising. That wasted time and money on those who always vote anyway, those who never vote, and those who (gasp) might vote Republican. Now the talk is of target “universes”: focusing resources on those who need just a nudge to come out and vote the right way.

It turns out that two principal campaign tasks—persuading swing voters, and turning out loyal but sporadic supporters—are made far more efficient if marital status is added to the mix, alongside such markers as sex, race, income and geography. That holds equally true when buying advertising alongside the right TV shows, and when leafleting selected homes in specific streets. Nationally, Page Gardner, a voter-registration expert, has crafted models that allow unmarried women to be found with great accuracy. Conservatives are still playing catch-up.

In November’s election for governor of Virginia—a race won narrowly by Terry McAuliffe, a Democratic fundraiser and member of Bill and Hillary Clinton’s inner circle—fully two-thirds of voters chosen for special attention by Democratic get-out-the-vote teams were unmarried women, says Michael Halle, a McAuliffe campaign guru. Unmarried women voted for Mr McAuliffe by a thumping 42 percentage-point margin over his Republican rival, Ken Cuccinelli, arguably handing him victory. (Married women backed Mr Cuccinelli by nine points.)

For an explanation, consider a third big development: the Republicans’ embrace of policies and slogans that might have been laboratory-crafted to upset and unite different types of unmarried women. A case in point is Mr Cuccinelli, whose candidacy dismayed establishment Republicans as much as it excited conservative activists. He is a shrink-the-government zealot: targets for his ire extend to municipal swimming pools (for crowding out the private sector). He is also a social conservative who opposes abortion, gay rights and no-fault divorce (as a state legislator, he proposed a bill to make divorce harder if one party disagreed). Poll-testing of different attacking strategies found single women outraged by Mr Cuccinelli’s social crusades. “Divorce was a big one,” Mr Halle fondly recalls.

Pragmatic Republicans know the party needs to tone down its social conservatism. But even so, it may struggle with singletons. Celinda Lake, a Democratic pollster, says single women of differing races, ages, classes and religiosity are united by a sense of fending for themselves. That makes them more likely to favour a strong role for government as a safety net. Republican appeals for the state to “leave us alone” sound different to women who are in fact on their own: especially those trying to support children.

The view from Virginia

That is reflected in Virginia. Tawana Bryant mobilises black voters for the National Association for the Advancement of Coloured People in Richmond, the state capital. Ms Bryant, who also ministers to single mothers at the Worship and Praise Deliverance Church, takes a conservative line on abortion. That used to prompt her to vote Republican in state and county elections. Economics changed her mind. Many single mothers work long hours in jobs that offer no health coverage and leave them struggling to feed their kids, Ms Bryant says. With Republicans vowing to cut food stamps and fight any expansion of subsidised health care, she argues, “You have to go with the Democrats.”

Republicans are not about to ditch their belief that children do better when their parents are married, nor their suspicion that Mr Obama’s “Hubby State”, as some call it, undermines strong families. But some think it would be wise to try harder not to repel single women. Katie Packer Gage, a co-founder of a Republican consultancy aimed at women, suggests some modest steps, among them understanding that the world looks “a little different” for unmarried women. Jennifer McClellan, a Democratic state legislator for some tough bits of Richmond, makes a related point: the single mothers she represents know that life is “complicated”, and want politicians who recognise that too. Such appeals to compassion and pragmatism have favoured Democrats to date. It would not kill Republicans to listen too.