The mood was convivial with food, drinks and campaign posters as more than 50 people gathered at a Jeb Bush campaign office in West Miami to watch the fourth GOP debate.

Ms Roxanna Greene, 53, an unemployed Bush supporter, said: "He may be the son of a president but he proved to me during Hurricane Andrew that he could roll up his sleeves and help people."

When Florida Senator Marco Rubio was on screen, Bush supporters heckled him, shouting "He's so dumb", or "He looks like Spock!" in reference to the famous Star Trek character.

Indeed, an interesting clash is brewing in Miami - home to both Mr Bush and Mr Rubio - as they pit themselves against each other in a bid for the White House.

While both made their respective presidential announcements there, the election campaign has made one fact very clear - the city is not big enough for both of them.

Experts have said that - while not a sure-win strategy - the conventional wisdom is that a candidate from Miami might have the support of voters from the key state of Florida, which would give the candidate an edge in the primary election as rivals tussle for the Republican nomination.

The problem for Mr Bush and Mr Rubio is that whoever loses the Florida primary in March also loses the ability to make the argument that he can deliver the state in the election.

Associate professor of political science Michael McDonald from the University of Florida said: "It probably helps a bit to have a candidate who is running from a state that they have represented in the past, though we can point to a lot of counter examples, but if Florida comes down to a close election, then you might be looking for any little bit of edge that you might be able to get."

Mr Bush definitely started off with the upper hand, having been governor of Florida from 1999 to 2007 and generally credited with growing the economy and bringing about school reforms.

"Most people think Jeb did a good job as governor," said Associate Professor Joseph Uscinski of the department of political science at the University of Miami.

But, of late, his stock has been falling with each poor showing on the national debate stage. Recent staff cuts in his campaign headquarters in Miami also signal some hiccups in the Bush campaign.

Law student Frank Carrasco, 30, said: "I used to be a supporter of Jeb Bush but have recently shifted my support to Marco Rubio... Bush hasn't been able to make his case to the American public in his debates of late...

"I think Jeb Bush would govern the United States well, however my concern is not just who will govern well, but who will be able to defeat the Democratic opponent in the election," he added.

A poll done in July among Miami-Dade county voters showed Mr Bush has 35 per cent of support, compared with 25 per cent for Mr Rubio.

The two, however, were the only Republicans with support in the double digits.

Even among older Cuban-Americans who are generally Republicans, and should identify with Mr Rubio's humble beginnings, the choice between the two is not clear.

While retiree Julian Dominquez, 83, feels Mr Rubio is the embodiment of the American dream, as his parents were a maid and a bartender, another retiree Adolfo Diaz, 66, said: "Jeb is my friend. Jeb is against communism."

Not only are the two Miami natives vying for votes, but for funds and endorsements as well.

While fund-raising is a nationwide endeavour, it often starts at home for candidates and the two are certainly sharing the same pie.

"Jeb can feed off his brother and father's donor network, which probably injures Rubio a little bit," said Prof Uscinski.

Mr Bush raised US$13.3 million (S$18.8 million) in the third quarter of this year, compared to Mr Rubio's US$5.7 million.

There might be street signs in Miami that claim "Florida is Marco Rubio Country", and supporters who insist Mr Bush will win the state - but the jury is still out.

Competing for the hometown crowd at the recent Sunshine summit in Orlando, both packed the conference hall and spoke of a strong American military, bringing back family values and shrinking the government.

As Prof Uscinski said: "I'm not convinced their messages are entirely different at the moment. I don't see a lot of daylight between the two."