Five reasons Marco Rubio may stumble

After a third solid debate performance and a skilful evisceration of Jeb Bush on Wednesday night, it's Florida Senator Marco Rubio's turn to step into the national spotlight. But can he withstand the glare?

Mr Rubio has had a lot going for him so far. He's run a slow, but steady campaign that has been relatively error-free. Although his campaign fundraising efforts have lagged behind candidates like Texas Senator Ted Cruz and former Florida Governor Jeb Bush, he's been frugal with his spending and has more cash on hand than many of his competitors. And he's attracted the critical eye of the New York Times, which in a Republican nominating battle is better than any endorsement.

But while Mr Rubio is the talk of Washington right now - and the new favourite among those who wager on political fortunes - there are still a great many potential stumbling blocks and possible pitfalls that he must navigate if he wants the nomination. While he and his supporters are saying all the right things about not getting ahead of themselves and taking the race one step at a time, it's a political truth that it's a lot easier to climb the hill than stay on top.

And in case anyone had forgotten, there are still three months until Iowa kicks off the nomination process - a very long time to try to maintain momentum in a topsy-turvy race.

So how could Mr Rubio stumble on his way to the prize?

Scott Walker couldn't take the heat as a Republican front-runner

Until you've been in it, you haven't been in it

Remember Wisconsin Governor Scott Walker? He shot to popularity back in January following a much-heralded speech at a Republican gathering in Iowa. He was viewed as the candidate who could unite the party establishment and the rebellious grass-roots. He was proven winner on the campaign trail and many people's pick as the eventual nominee. And his presidential bid failed. Spectacularly.

Mr Walker wilted under the full force of media scrutiny, as the day-to-day grind of a national campaign took its toll. The Wisconsin governor learned the hard way that presidential politics is the big leagues, and it's unforgiving of weakness or uncertainty.

Could a similar fate be in store for Mr Rubio? The sum total of his campaign experience is winning a US Senate seat as a significant underdog in 2010, five Florida elections to the Florida State House of Representatives (three of which were unopposed) and one race for local office in West Miami - a job he quit midway through his first term.

This seems to be the year of the political neophyte in Republican presidential politics, but when it comes to running a successful national campaign, experience usually matters. Mitt Romney won on his second try in 2012, as did John McCain in 2008. George W Bush had his family's vaunted machine behind him in 2000, and in 1996 Bob Dole won the nomination on his third attempt.

Rubio has avoided tough questions about his past - so far

Skeletons in the closet

On Wednesday night Mr Rubio shrugged off a question about his past personal and campaign finances dealings by turning it into a shot at media bias and then a rumination on his humble upbringing.

While it worked in the context of a debate where every panel question was viewed with scepticism or outright hostility from the audience, Mr Rubio may not always be so fortunate. And some of the issues aren't just about poor personal financial planning but tilt toward allegations of corruption

As the New Yorker's Ryan Lizza points out, there's a litany of questionable actions in Mr Rubio's past - such as failing to report a home equity loan from a political supporter, double-billing the state and the Republican Party for travel expenses and using political funds to pay for seemingly personal expenses like car repairs and groceries.

Mr Rubio also was a friend and real-estate partner with David Rivera, a Florida politician and former congressman, who has been fined more than $16,000 [£10,500] for government ethics violations and is still under federal investigation for related criminal acts.

"Rubio is about to go through a period of much more intensive media scrutiny," Mr Lizza writes. "Complaining about media bias won't be enough to get him through it."

Jeb Bush has a large campaign bank account - and could use it to take down Rubio

Big guns taking aim

Media scrutiny over questionable dealings in Mr Rubio's past may be the least of the Mr Rubio's concerns, however. Perhaps the greatest threat to the Florida senator is the barrage of negative advertising that is likely headed his way.

Although Mr Bush's campaign is scuffling, its supporting "independent" political action committees are still flush with cash and have identified Mr Rubio as a clear and present threat. Bushes seeking the presidency can hit hard - as opponents of George W Bush and George HW Bush will attest.

During a meeting with donors last weekend, Bush's campaign team circulated portions of a memo that called Mr Rubio "a risky bet" and contained an outline of possible lines of attack on the senator - including some of the aforementioned financial issues.

In an interview with Bloomberg Politics, Mike Murphy - who heads a pro-Bush PAC that has raised more than $100m [£65m] - warned "if somebody takes a poke at Jeb, we're capable of poking back."

A slide from a Bush campaign presentation outlines Rubio's perceived weaknesses

He said if this is Mr Rubio's "golden moment," he's going to be under sharp scrutiny for his flimsy record and "mystery donors".

"The second and third look are going to be very tough on Marco Rubio," he added.

Then there are the Democrats, and Hillary Clinton's team in particular, who are closely monitoring Mr Rubio's rise and, according to Politico's Daniel Lippman, committing more resources to investigating his past.

• Is Rubio Hillary Clinton's worst nightmare?

"It's a massive project," he writes. "Info is scattered across his time in various governments, and researchers are cross-referencing the data."

Democrats know that the best time to define opponents in a negative way is before they become better known by the general public. Barack Obama's campaign successfully painted Mr Romney as an out-of-touch millionaire in 2012, for instance, and it's a technique Mrs Clinton will likely follow in 2016.

Donald Trump and Ben Carson have dedicated followers who won't be quick to defect to Rubio

The outsiders haven't left

While many in media and politics may be tired of real-estate tycoon Donald Trump's bombastic routine and retired neurosurgeon Ben Carson's restrained, often opaque delivery, they're still the Republican front-runners until proven otherwise.

The first scientific poll of debate viewers found that while Mr Rubio performed well, a plurality thought Mr Trump was the winner. An Ipsos survey conducted in the days leading up to the debate show Mr Trump and Mr Carson in a virtual tie with 29% and 27% respectively, while Mr Rubio sits in fourth place with 6%, behind Mr Bush (9%) and just one point ahead of Texas Senator Ted Cruz.

In other words, Mr Rubio has a long way to go if he's going to turn his good press clippings into tangible support. In addition, those backing anti-Washington outsiders like Mr Trump and Mr Carson seem unlikely to be lured away by a polished performance from a US senator perhaps best known for his early support of comprehensive immigration reform.

Mr Trump's numbers have been solid for months, and Mr Carson's still appears to be on an upward trajectory. While Mr Rubio may pique the interest of party donors and voters who are currently supporting other establishment candidates like Mr Bush or Ohio Governor John Kasich, that currently accounts for well under half the Republican electorate.

Mr Rubio hasn't made many campaign visits to Iowa so far this year

A campaign built to succeed?

Although Mr Rubio has more than $10m in cash on hand, his campaign does not appear to have made the kind of infrastructure-building efforts on the ground in early-voting states like Iowa and New Hampshire that usually translate into success on election day.

Mr Rubio himself has only spent 20 days in Iowa and 14 days in New Hampshire since 2013 - much fewer than many of his competitors. Most of his efforts have been focused on national fund-raising tours, according to the Washington Examiner's Byron York, hence the concerns over his absenteeism in the US Senate that gave rise to Mr Bush's ill-fated debate attack.

"Right now, Rubio has managed to be the candidate who is often absent from his day job while still not spending that much of his time with voters," York writes.

In South Carolina, third in line in the nomination process, Mr Rubio has yet to open a campaign office, causing some Republican operatives there to wonder if he takes their state seriously.

Rubio campaign officials has replied to criticisms like these by saying they're biding their time and shepherding their resources, aiming to peak when voting begins in February.

Sometimes, however, it's good to capitalise on advantageous circumstances - and to do so in places like Iowa and New Hampshire requires having a robust campaign team in place.

Mr Rubio could be biding his time, but there's the chance that his time is now.

Candidates in (and out of) the Republican presidential field