In Phoenix earlier this year, I met Tony Valdovenos, one of a small army of on-the-ground volunteers deployed by Barack Obama’s campaign to register new voters in Arizona.

After just a few weeks on the job, he claimed to have signed up about 100 potential voters, all Hispanics. He was especially motivated, he told me, because as someone brought into the country by his illegal immigrant parents, the one voter he can’t register is himself. “At the end of the day, this is very personal for me,” he said.

Not only is it not possible for Valdovenos to vote, but the new legislation has also made it too expensive for him to study. Illegals can no longer get the same financial support for state universities that is available for locals. As a result, he has dropped out of university in Phoenix because he can’t afford the $80,000 upfront cost for a four-year degree.

Arizona has become ground zero in the heated immigration debate in the US. This has happened in the wake of the tough 2010 legislation making police determine the immigration status of anyone they stop or search, and restricting state services for illegals. In the process of securing the Republican nomination, Mitt Romney took a tough line on illegal immigration and even publicly supported the Arizona law. It is something that Obama’s team is determined no one forgets in the run-up to the November election.

Heading into the poll with low approval ratings on the economy, President Obama is deploying other weapons in his fight to keep the White House and few are more important than the Hispanic community. Hispanics are at the forefront of an extraordinary ongoing racial makeover of the US population that has been occurring over the past four decades and which is literally changing the face of the country. In the process, this community has the potential to transform US politics too.

According to the 2010 US Census, people identifying themselves as Hispanic accounted for more than half of the country’s population growth of 27.3 million in the previous decade, increasing at four times the rate of the rest of America. The Hispanic population, a definition which covers anyone who traces their parents or ancestry to Spanish-speaking countries, reached 50 million, or one in six Americans, by 2010. By 2050, on current projections, they could make up one-third of the population. Much of the increase is homegrown: children born in the US to Hispanic parents, millions of whom arrived in the country illegally and remain in a legal limbo land, without any way to gain citizenship.

Some Republicans, like former president George W. Bush, also a former Texas governor, embrace Hispanics, but the party’s nativist wing has prevailed in recent years, rejecting proposals to find a way for illegal immigrants to gain citizenship.

In place of the pro-citizenship plan, known as the Dream Act, which is now part of the Obama administration’s platform, the loudest Republican voices are identified with mass deportation, summary arrest and electrified border fences.

Although demography is not destiny in politics, Hispanics have flocked behind President Obama, largely overlooking his own failure to push the DREAM Act when the Democrats controlled Congress in the two years to 2010. In the latest polls, Hispanics support Obama over Republicans by a margin of more than two to one.

The split over Hispanics underlines a broader, disturbing trend, a wholesale re-racialisation of US politics, with whites, especially men, clustered around the Republicans, while African Americans and immigrants find a home with the Democrats. If sustained, such a trend should favour Democrats, as the white share of the population is shrinking. States like Texas, which is already more than a third Hispanic, could start to lean Democrat by 2020 on present voting patterns. This comes after two decades as a conservative bulwark.

“The nativist message you hear when Republicans talk this way extends to all immigrant groups,” Jon Huntsman, former Republican candidate told me over coffee. “I am not sure that Republicans have figured that out yet.”

The immigration split between the Republicans and Democrats has dangers, including the possible Balkanisation of the political landscape with policies that entrench the racial divide. Democrats could fall into the trap that engulfed the Labor Party in Australia, embracing immigrant communities while alienating the traditional working-class voter.

The Obama campaign is going to extraordinary lengths to find and get Hispanics to the polls. In recent months Spanish-language television ads ran in four states—Florida, New Mexico, Nevada and Colorado. Each had different voiceovers and scripts to capture the accents of the variety of Hispanic communities, ranging from Puerto Rican, to Cuban, to Mexican.

In the meantime, with the primaries out of the way, Romney is pondering how to regain the ground he has lost with Hispanics. He is being helped by Marco Rubio, the Republican senator from Florida, who is preparing a new version of the DREAM Act containing provisions for guest workers, which he hopes will be acceptable to the right. The charismatic Rubio, who is of Cuban origin, has unusual credentials to build bridges on a difficult issue: he is both a Tea Party favourite and also someone who has long tried to moderate the tone of the immigration debate.

In some ways, the problem is beginning to solve itself. The economic downturn, tighter border controls, and increasing prosperity in Mexico brought net immigration to a standstill by 2011, according to the Pew Research Center. The number of illegal immigrants from Mexico has also fallen, from 7 million in 2007 to 6.1 million in 2011. But this historic turnaround has not yet filtered into the political debate. The question for Romney is not whether he shifts his position on immigration, but how and when.