Both of them have a Spanish-sounding surname. Both have a moving story of poor parents coming to America for a better life. And both recently spoke to the American people from a primetime slot at their party's national convention.

But San Antonio mayor Julián Castro and Florida senator Marco Rubio are far from being on the same side. Instead they symbolise the almighty battle for the Hispanic vote between Republicans and Democrats in the 2012 election.

Each has been selected as a sort of party champion, sallying forth to do battle for the support of America's 50  million Latinos, whose votes could decide whether former Massachusetts governor Mitt Romney wins the Oval Office or President Barack Obama secures a second term.

The contest has pitted two of the youngest rising stars in American politics against each other. Castro is just 37, yet already has a major national profile, prompting comparisons with Obama's own path to the White House. Meanwhile Rubio, 41, is a Tea Party favourite often touted as a future Republican leader who was even widely rumoured to have been considered for Romney's vice-presidential shortlist this year.

Both men are the children of immigrants – from Mexico and Cuba respectively – and both gave widely praised speeches that wowed their fans. Yet, mirroring the many splits in American Hispanics' own communities, that is where the similarities end. Rubio is a darling of the right wing of the Republican party and beats the drum for slashing government spending. He is staunchly anti-abortion and has said that gay marriage goes against his Christian faith.

Castro, meanwhile, has served as the grand marshal in his city's gay rights parade, praises the role of government in society and is the child of a leftist civil-rights-campaigning single mother. Each man therefore represents a radically different vision of what his party thinks will appeal to Hispanic voters. "Both parties need to show that they are open and attractive to Hispanic voters, but they are not at all a unified bloc," said Professor Scott McLean, a political scientist at Quinnipiac University.

So far it is a battle that Castro and the Democrats have been winning. In 2008, Latino voters went for Obama by almost 70% and current polling – though showing a drop in support – still has the Democrat ahead of Romney by 63% to 28%. In the recent Charlotte party convention, Castro was given the all-important keynote address – the first Hispanic to fill that spot – but other Latino party bigwigs were given prominence too. Much was made of Obama's appointment of the first Hispanic woman to the Supreme Court in Sonia Sotomayor, and the president was also lauded for recent moves to lift the threat of deportation from millions of young illegal immigrants brought to the US as children.

It is that thorny issue of illegal immigration that is the Democrats' real strength. Though Obama is hardly liberal on the issue, the Republican party has moved far to the right, embracing controversial measures against illegal immigration in states such as Arizona. Yet one study has estimated that some 9.5 million people – the vast majority of them likely to be Latino – live in "mixed" households where at least one person is illegal. In such homes, harsh Republican rhetoric on immigration is always going to be offputting.

"That is the wedge issue that keeps Latinos from the Republican party," said Professor Gabriel Sanchez, an expert in Latino politics at the University of New Mexico.

Yet Republican strategists believe they have an ace up their sleeves in the party's embrace of conservative social values that are often shared by many Hispanics, who are often deeply Roman Catholic and family-oriented. Rubio and other leading Republicans, such as Texan Senate candidate Ted Cruz and New Mexico governor Susana Martinez, frequently tout their social values. But it has not been a winner overall, because Latino social conservatism often goes hand-in-hand with a belief in government welfare programmes as a way out of poverty for a demographic often much poorer than its white equivalent. This is especially true as the economic aftershocks of the Great Recession are still being felt while Republicans tout massive cuts to healthcare, education grants and food stamps.

"Who cares about two guys getting married when you want a job?" said Professor Stephen Nuño, a political scientist at Northern Arizona University. "In the end, a voter is not going to forget Republican hostility and that the party wants to deport their uncle."

Many experts believe the Republican party faces disaster in the future as Hispanics became an ever more vital part of the electorate. In 2008, some 10.2 million voted, a whopping 25% increase on 2004 (in the early 1990s, only 2% of the electorate was Hispanic). It is a number certain to leap again in 2012.

Recent surveys have shown that Hispanics represent more than half of all US population growth over the past decade. Due to a youthful population profile, they already represent one in four Americans under the age of 18. "The numbers are compelling," said Sanchez.

So, too, is the geography. One of the fastest areas of Latino population growth is the Republican heartland of the south. If the Republican party does not in some way attract Hispanics, it will face a withering of support even in the reddest of red states. The battle is already on for Texas – a solidly Republican state that is home to both Castro and Cruz. It is noticeable that Texas's Republican governor, Rick Perry, is already softer on aspects of illegal immigration policy than the Republican mainstream. But if the Republicans are to compete nationally as the voice of Hispanics, if Marco Rubio is to best Julian Castro on the national stage, then the Republican party itself will have to change. "It has to. But I don't see that that is going to happen for a very long time," said Nuño.