Kasha Nelson, 31, was only supposed to go hiking with a friend. But in the 45 minutes it took to drive from Taos to Ski Valley in northern New Mexico she'd changed her vote. For the last three elections she's voted Democrat, but this time she was leaning towards Romney. "In 2008 I got caught up in the whole hope and change thing, but I've lost total confidence in Obama's ability to get us out of this mess and I thought, with his business background, Romney might be a different breed of politician."

She shared her intentions with her friend, a woman in her 20s and in a long-term relationship, who promptly reminded Kasha that she had just had an abortion. Romney's election, and the supreme court nominations that come with it, her friend argued, could take away that choice and criminalise that action. Is that what Kasha wanted? Kasha, who was raised "very" Catholic in a trailer in southern Alabama, believes abortion is a necessary evil. She finds the Republicans' social conservatism a "real turn-off" but "has a lot of faith in the independence of the supreme court".

Until then, Kasha had become weary of the way all conversations about the election with her friends would start with discussions about making ends meet and then switch to abortion, contraception and gay marriage as soon as she mentioned she might vote for Romney.

She was beginning to feel like her life was stalling and was looking for a leader who could revitalise the economy. Once a bilingual teacher, she took a career break to do a creative writing course in Santa Fe, and was now struggling to get back into the job market. After we finished breakfast she was driving five hours to Denver to take up an office job paying $6,000 less than the teaching job she'd left a few years ago.

But when confronted with the direct experience of her friend and the direct consequence of her vote, she felt compelled to reconsider. "I went back and looked up where Romney stood on the issues on his website and I just decided I couldn't vote for that. My vote couldn't just be about money and jobs."

There are (to adopt Romney's phrase) "binders full of women" who are turned off by the Republicans' policies on reproductive rights. But those binders become even thicker when marital status is taken into account. A summer poll gave Obama a 46-point lead among single women like Kasha and her friend. And almost every year there are more of them. Thanks to feminism and the increased educational opportunities, equality laws and reproductive choices that have come with it, women are getting married and having children later in life. In 1970, unmarried women comprised 38% of the population; today they are 47%. Kasha's mother got married when she was 19 and had Kasha shortly afterwards. Kasha has other options. She'd like to keep them.

It's just one of the examples of the many ways in which Americans live, love, move and behave that is having an impact on the nation's electoral landscape. The US is becoming less white, less religious, more urban and diverse in its living arrangements, and migrating to the south and west. Some of these changes, like the growing strength of the Latino vote or an ageing population heading for the sun belt, are demographic. Last year, for the first time ever, the majority of babies born in America were not white, presaging a near future where white people will be a minority. Others, like the propensity of people to marry later and for women to remain single longer, are social. Some of these changes are happening very quickly: the proportion of children born out of wedlock from 2006-2010 was double that born in 2002. Others take more time: during that same time span the median age at which American women married increased by less than a year. Many are not immediately obvious. The fastest growing "religious group" in the US is those with "no religious affiliation". In 1944 they were one in 20; in 2004 they were one in seven; by 2024 they are projected to be at least one in five. But all have political consequences. America is changing far faster and more thoroughly than its electoral rhetoric, strategies and alliances can keep up.

"The tectonic plates of American politics are shifting," argues Ruy Teixeira in a 2010 paper Democratic Change and the Future of the Parties for the liberal thinktank, the Center for American Progress. "A powerful concatenation of demographic forces is transforming the American electorate and reshaping both major political parties. And, as demographic trends continue, this transformation and reshaping will deepen."

'Latinos became more moderate'

In few places is this clearer than New Mexico, which has gone from the swingiest of states to being safely Democratic in just three election cycles. In 2000, the outcome in New Mexico was closer than Florida: Al Gore took it with a majority of just 366. In 2004 the Republicans won it back with a slightly healthier 5,988 lead. Then in 2008 there was a rout. Obama reclaimed it with a 15-point margin and a lead of 125,590. This year it is not even being contested. The last four polls there have Obama leading by double figures.

The most obvious explanation for this would be the rising number of Latinos. Between 2000 and 2010 the state's Latino population grew 24.6%. Between 2000 and 2008 the number of registered Latino voters leaped by 44% and the number of actual Latino voters jumped by 51%. At just over a third New Mexico has the highest proportion of Latino voters of any state in the country.

But that explanation, on its own, would be incomplete and incorrect. The number of Hispanics were growing in 2000 and 2004, yet the Republicans won the state back and increased their share of the Latino vote. The numbers continued growing and the state still elected a Republican governor (Susana Martinez, who is Latina) in 2010. So there is nothing inherent about a growing Latino population that makes a Democratic outcome inevitable. True, Latinos tend to be poorer and poor people tend to vote Democrat. But Latinos are also more likely to be religious and against abortion than the nation at large too. Republicans have the only two Latino governors in the country – Martinez and Brian Sandoval in Nevada. After this election cycle they will have more Latino senators than Democrats. Of the five states with the highest proportion of Latinos two, California and New Mexico, are safely Democratic. Two, Arizona and Texas are, for now, safely Republican, and one, Nevada, is a swing state.

So demography is not electoral destiny. People vote their interests not their identities and parties mould their platforms to appeal to people's interests. There is nothing inevitable about either. Women are evenly split on the issue of abortion; polls showed Catholics more likely to demand employers provide healthcare plans that cover birth control at no cost than Protestants. Until the 1980 Republicans were more likely to be pro-choice than Democrats. The complicating factor is that quite often those interests are intertwined with their sense of identity. Wealthy African Americans overwhelmingly vote for Democrats not because they will be better off financially, but because they are turned off by the Republicans' racially charged rhetoric.

"A decade ago there would have been a large number of Latinos who saw themselves as conservative. But they became more moderate as the Republicans moved to the right," says Gabriel Sanchez, a professor at the University of New Mexico in Albuquerque. The decline has been quite dramatic. In 2004 Bush got 44% of the Latino vote; in 2008 McCain netted just 30%; a poll released earlier this week by Latino Decisions suggest just 24% will vote for Romney.

The reason New Mexico has turned Democrat isn't just because it became more Latino but because Republicans simultaneously became perceived as more xenophobic. "Republicans have really leveraged the anti-immigrant rhetoric to appeal to their white nativist base, and in the short term that might work nationally," explains Sanchez, who also acts as the director of research for the polling organisation Latino Decisions. "But in the long term it will have negative effects. They are seen not as anti-immigrant but anti-Latino."

'This election is a critical juncture for the Republican party'

California is used as the cautionary tale. Between 1952 and 1988 it voted Republican every time bar one. In 1992 Clinton won it with 46% of the vote but it still had the potential to swing back. In 1994 came Proposition 187, a Republican-backed measure to bar undocumented migrants from healthcare, public education and other services in the state. They won the vote (though the law was found unconstitutional) and lost Latinos. Now, like New Mexico, it's safely Democratic. "Research show that Latinos who came of voting age during that time are less likely to ever vote Republican again because that is when their image of Republicans was formed. Latinos are not in the bag for Republicans yet. But it could get that way if Republicans don't change." If it did, Texas and Arizona would, sooner or later, go the way of New Mexico and California.

This is not just a problem locally, but with the entire Republican electoral strategy since the end of the civil rights era – the last time in fact that there was a seismic shift in the nation's electoral allegiances. Back then Republicans realised that if they could shed their reputation as the party of Lincoln they could peel off a whites in both the south and northern suburbs with a subtle appeal to racial animus. It worked. In 1976, Jimmy Carter won all the former confederate states bar Virginia. By 2000, Al Gore won none, even though he and his father had represented one of them (Tennessee) as senators.

Richard Nixon explained the plan to his chief-of-staff, Bob Haldeman, who wrote it in his diary. "You have to face the fact that the whole problem is really the blacks," Nixon told him. "The key is to devise a system that recognises that while not appearing to."

When white people made up around 85% of the voters that made sense. But over the past three election cycles alone the white vote has dropped from 80% in 2000 to 74% in 2008. With 50,000 new Latinos becoming eligible to vote every month, the long-term trend for that white share of the vote will be downward, which means the more black and Latino voters the Republicans alienate, the more white voters they need to replace them. "The demographics race we're losing badly," said Republican, senator Lindsey Graham, acknowledging the problem. "We're not generating enough angry white guys to stay in business for the long term."

They are trying their best. Romney has a 20-point lead among whites – the biggest racial gap for almost 30 years. Meanwhile, Joe the Plumber, now a Republican congressional candidate in Ohio, has an original plan for immigration reform. "Put a fence on the damn border and start shooting," he says.

But sooner rather than later Republicans will either become more inclusive or less viable. "This is the last time anyone will try to do this," one Republican strategist told the National Journal. And Republican consultant Ana Navarro told the Los Angeles Times: "Where his [Romney's] numbers are right now, we should be pressing the panic button."

Republicans' stance on reproductive rights, like their southern strategy on race, was engineered during the early 1970s with electoral advantage in mind. Photograph: AP

For in an increasingly polarised electorate loyal constituencies make a big difference. In swing states like Nevada, Florida, Virginia and North Carolina, white people comprise less than two-thirds of the population. In Colorado, Latinos make up 20%. Even in states like Ohio, Wisconsin or Iowa, where Latinos make up between 3% and 5% of the population, they can make a difference in a close-run contest.

"This election is a critical juncture for the Republican party," says Christine Sierra, heads UNM's Southwest Hispanic Research Institute. "Latinos would be a more diverse voting bloc if there were reasons to vote Republican. But without those reasons its possible they could become as cohesive a voting bloc as African Americans."

Republicans' stance on reproductive rights, like their southern strategy on race, was engineered during the early 1970s with electoral advantage in mind. In 1969 George Bush Sr, then a Texas congressman, told the House: "We need to make family planning a household word. We need to take the sensationalism out of the topic so it can no longer be used by militants … who are using it as a political stepping stone." That same year Nixon said: "No American woman should be denied access to family planning assistance because of her economic condition." But a couple of years later, as Jill Lepore points out in a New Yorker article, Patrick Buchanan wrote a memo to Nixon advocating using the abortion issue to woo the Catholic vote. "If the president should publicly take his stand against abortion as offensive to his own moral principles … then we can force [Ed] Muskie [a failed Democratic presidential candidate in 1972] to make the choice between his tens of millions of Catholic supporters and his liberal friends." The next week Nixon spoke of his "personal belief in the sanctity of human life – including the life of the yet unborn".

This has served them better among evangelicals than Catholics, but it is causing considerable damage to their reputation among the growing number of educated women and unmarried couples.

"I just think Republicans want to take us backwards to another time," says Erica, 37, a school teacher who lives in Taos, New Mexico, where she raises her child alone. "When they're talking about family values I don't think they're talking about a single mum who's juggling child care and work. I don't think they can related to a single woman in her thirties with some life experience. When you go to Planned Parenthood they don't offer you an abortion as soon as you walk through the door. They talk through your options."

"They just seem so tunnel-visioned," says Erica, who was raised evangelical in Minnesota. The episode when Rush Limbaugh referred to Georgetown student Sandra Fluke as a "slut" and a "prostitute" after Fluke spoke in support of mandating insurance coverage for contraception, she says, is a case in point. "That suggests a larger attitude towards women. I don't know if Romney agrees but a large segment of his party does and he didn't come out and say that's crazy."

Some progressives believe these trends spell the death knell for the Republican party in its present form. "The Democratic party will become even more dominated by the emerging constituencies that gave Barack Obama his historic 2008 victory," argues Teixeira, author of The Emerging Democratic Majority. "While the Republican party will be forced to move toward the centre to compete for these constituencies. As a result, modern conservatism is likely to lose its dominant place in the GOP."

That's too deterministic. For a start there are countervailing trends. The migration of Americans south and west over the years and the reapportioning of electoral college seats to accommodate those population movements has helped Republicans. If Obama won the same states this year as he did four years ago he would have six electoral college seats less because the population in the states he won has not risen as fast as in the states he lost. Demography alone over the last decade has handed Republicans the equivalent of Iowa.

Moreover, most of the groups Republicans are losing are less motivated than those they have gained. Only 71% of Latinos, 76% of the unmarried and 78% of the non religious say they will definitely vote. Conversely 86% of whites, 87% of Protestants and 88% of married people say they will. This is why there is such a discrepancy between how Obama fares with registered voters compared to likely voters and why the 'ground game' will in the coming weeks will be so crucial. For all the emphasis on women voters Obama is tied with women likely voters overall and trails white women by 9 points.

For the Democrats to take advantage of these changes they have to give people something to vote for and then get them to vote. That's not necessarily as easy as it sounds.

Still unable to bring herself to vote for Obama, Kasha cast her ballot early for Libertarian candidate Gary Johnson. "I'm totally disgusted with the two-party system," she says. "We don't have a good choice, and I don't want to want to compromise. I don't care if the guy I vote for doesn't get to win. I've had enough of just voting out of fear for the other side."