Image copyright AP

Hillary Clinton has strengthened her position as the Democrat to beat in her quest to become the country's first woman president. But the biggest obstacle in her path to victory could be a young senator who few outside Florida know much about.

If Republican strategists were to assemble their ideal presidential candidate in a factory, a product resembling Marco Rubio would come rolling off the conveyor belts in a perfect package.

Young. Tick.

Clear communicator. Tick.

Good on television. Tick.

American Dream life story. Tick.

Strong Hispanic appeal. Tick.

Keeps gaffes to a minimum. Tick.

While other Republicans have gained far more attention, the 44-year-old Cuban-American has been quietly impressive.

Too quiet to trouble the leaders, one could joke. In the Republican race, he lies a distant third in an average of national polling by RealClear Politics, a long way behind Donald Trump and brain surgeon Ben Carson.

But for some who have closely followed US elections for years, he is the "real" frontrunner.

The top six Republicans

Image copyright AP

Donald Trump 27% (left)

Ben Carson 21% (middle)

Marco Rubio 9%

Ted Cruz 8% (right)

Jeb Bush 7%

Carly Fiorina 5%

Source: RealClear Politics

He probably has the best chance of anybody of being nominated, says Stu Rothenberg of the Rothenberg Political Report.

"If you gave me [truth drug] Sodium Pentothal and asked me who I thought had the best chance to be the presidential candidate, I would say Marco Rubio. He has charisma, pizzazz, youth and he's a terrific speaker."

But the race is still uncertain and there are question marks about his substance and experience, he warns. Some Republican voters see another Barack Obama, a 40-something senator who got to the White House and was ill-prepared.

Yet Rubio's is the name on the lips of many political insiders. But why, with Trump and Carson so far ahead in the polls?

His path to victory would best be described as slow and steady.

In January as the primary contests loom, Republican voters will ask themselves who can really be president, says Rothenberg.

Those who have backed "outsider" candidates Trump and Carson as an expression of anger with the party's elite will turn to more mainstream contenders, he thinks.

"There could be boredom with Trump, there could be self-inflicted wounds but most likely it will be voters starting to evaluate candidates differently. They have always done that in the past. I can't guarantee they will this time but if they do the field will look much narrower."

Image copyright Reuters Image caption His polling numbers are best described as tepid but Rubio is still hotly tipped

Image copyright AP Image caption Rubio connects with young Republican voters

They don't want to nominate another "old, white guy" if they want a chance, he says, and Rubio's background would put him ahead of the well-financed but struggling ex-Florida governor, Jeb Bush.

A lot of candidates like to cast their life story as a rags-to-riches American fable, but Rubio has a strong and compelling case to make.

Born in Miami to Cuban immigrants, he lived in Las Vegas for a while as a child, where his father was a barman and his mother a housekeeper. After high school in Miami, his degrees in political science and then in law amassed a $100,000 student debt that he only paid off in recent years.

He scaled the political ladder in Florida at speed, culminating in a Senate seat in 2010 after a race he was never expected to win.

"When I entered that race, the only people who thought I could win all lived in my home. Four of them were under the age of 10," he has said.

Where Rubio stands

Immigration: Used to be pro-citizenship but now takes a harder line

Iran: Will undo nuclear deal "on day one"

Pro-life: Right to life "trumps virtually any other right"

Healthcare: Repeal Obama's signature law that extends insurance to millions

Climate change: No evidence that humans responsible

China: Promises to "get tough" about human rights

Gun laws: Violence is due to mental illness

Full list of policies on his campaign website

Socially conservative and hawkish on foreign policy, he toes the mainstream party line and appeals to both Wall Street and Tea Party factions of the party. But his powers of address arguably have no equal in the race.

A former Florida House Democratic minority leader, Dan Gelber, once said: "When Marco Rubio speaks, young women swoon, old women faint and toilets flush themselves."

These skills did desert him when it mattered, however. A very dry mouth when delivering a high-profile TV speech sparked water-drinking jokes and memes.

That awkward moment will be forgotten if he wins the Republican nomination, and many believe he is the candidate that Hillary Clinton, if she wins the Democratic race, will fear the most.

It's not just her appeal to Hispanics that he could dent but he offers a generational contrast with a woman who has been in politics for 20 years and who will be 69 come election day.

Image copyright AP Image caption Grilling burgers in Iowa is one of the campaign customs

Clinton's husband Bill is reportedly among those concerned at this threat, and her team have long been preparing a detailed attack plan on how to neutralise his appeal. His harder line on undocumented migrants getting citizenship, a reversal in policy, is sure to figure.

Rubio's connection with young people is the key to Hillary's vulnerability, says Susan MacManus, political science professor at the University of South Florida in Tampa, who has followed the senator closely for many years.

He talks their language, listens to their music and has no wealth, she says. The fact he has had debts of his own adds to this kinship.

"Here is a guy who understands their economics. Hillary needs to hold the young vote Obama won and she can't lose any of that."

Others believe, however, he's still a long shot. More like a vice-president pick, says political scientist Shaun Bowler, because he lacks cash.

"The difficulty for him is that there are still other strong - and stronger - candidates who will be left in the race even if or when Trump, Carson and others drop out.

"Rubio is doing OK but he doesn't have the resources these other candidates have."

That could yet change.

A polished performance by Rubio in the next Republican debate on Wednesday could convince Jeb Bush's donors to back the other Florida horse.

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