The United States has 52 million Hispanic citizens, including 24 million who can vote -- and many are not happy about either Barack Obama or Mitt Romney's immigration policies.

In a too-close-to-call election which climaxes on November 6, the issue has shot up the agenda for both candidates, as they battle to snag every single vote out there.

Republican challenger Romney, who has criticized a legislative bill which would grant rights to undocumented immigrants, has barely 20 percent among Latinos, according to a study by polling body Latino Decisiones.

Obama has some 71 percent -- but Hispanics are also disappointed in him for failing to fulfill a 2008 vow for comprehensive immigration reform, even if he announced measures to partially legalize young Hispanics a few months ago.

The Obama poll number astonishes Gabriel Sanchez, a political science professor at the University of New Mexico. "How is that possible, with the disappointment in immigration reform?" he asked rhetorically.

"I think it's because they have a choice between Obama... who speaks a lot but doesn't do much, and the 'self-deportation' as an immigration policy model," he told AFP.

He was referring to a phrase used by Romney during the Republican primaries earlier this year -- when he was notably more hardline on immigration than he has been since clinching his party's nomination.

Critics take the concept of self-deportation as implying that, if elected to the White House, Romney would make conditions for undocumented immigrants so difficult that they themselves would decide to leave the country.

Hispanic voters, who represent 11 percent of the electorate, rank immigration as one of their five chief concerns, albeit behind the economy and jobs, according to the Pew Hispanic institute.

While it is not the top concern, it could nonetheless tip the balance, notably in US states with a large Hispanic population, including battleground states Nevada, Colorado and Florida.

Republican Latinos accuse Democrats of exploiting the issue "to divide the Republican Party and... Hispanic communities," according to Alci Maldonado, head of the Republican National Hispanic Assembly.

She notes that it was Republicans who presented a vast immigration reform plan under President George W. Bush, which failed to make it through the Senate in 2007.

And today Republicans "are facing the consequences of the negative environment which they created for Latinos," Jody Vallejo, a sociology professor at the University of Southern California, told AFP.

This was the case "in particular in states like Arizona, where Latinos are targeted by Republican lawmakers," he added.

Arizona is infamous for a controversial law allowing police to check the identity papers of anyone stopped when they have a "reasonable suspicion" about their immigration status.

And while second- or third-generation Latinos don't have a problem with their papers -- typically they have US citizenship by then -- laws like Arizona's impact on the whole community.

"It doesn't matter if you you've only just arrived or if you're American of Hispanic origin; they will ask for your papers if you look like a Mexican," said Sanchez.

Moreover Latino Americans have emotional ties with undocumented Hispanics, and are "particularly concerned when they are faced with a party which attacks immigrants," said Vallejo.

Obama announced measures in June to stop the deportation of children of undocumented immigrants, and grant them two-year residency and jobs permits as long as they fulfil certain conditions, including having no criminal record.

The measures -- slammed by Republicans as pre-election opportunism -- amount to a watered-down version of the DREAM Act, which has been awaiting approval by the US Congress for a decade and about which Romney is skeptical.

The DREAM Act -- short for the Development, Relief, and Education for Alien Minors bill -- holds out hope of US citizenship and a residence permit to undocumented immigrants who arrived in the US before age 16.

It is unclear what will happen to the DREAM Act if Romney wins the White House. At one point he pledged to veto it, but has rowed back on that threat in recent months as he courts Latino voters.

But for Sanchez, "the question today is not whether Romney can attract enough Latino votes, because that will not be the case, but whether Latinos will turn out to vote to re-elect the president."