Senator Marco Rubio shakes hands as he leaves the Foreign Policy Initiative breakfast on August 14, 2015 in New York
Washington (AFP) - The Republican candidates in the 2016 presidential race are, by and large, not a very cheery bunch.
Donald Trump has disparaged more people than King Lear, Chris Christie has reveled in verbally brawling with hecklers and Lindsey Graham appears perennially on the cusp of declaring war.
On the campaign trail, each candidate has inveighed against the ills of modern America, which -- as they tell it -- is apparently just one or two executive decisions away from sinking irrevocably into a moral morass and global irrelevance.
After seven years of seeing a Democrat in the White House, that rhetoric fits the Republican mood pretty well.
But one GOP candidate has remained relatively chipper.
In fact, Marco Rubio, the youthful Florida senator of Cuban extraction, has made optimism the leitmotif of his campaign.
"I believe America is the greatest nation on Earth!" his campaign website screams.
"We have it in us to make the 21st century a time for your children and grandchildren to be the freest and most prosperous Americans that have ever lived."
To be sure, Rubio still bashes President Barack Obama and likely Democratic candidate Hillary Clinton. But at the risk of being compared to Obama, Rubio's message has broadly been one of "hope and change."
So can a youthful Hispanic candidate's positive message resonate with a Republican electorate that is largely old, white and angry as hell?
"Trump is getting a lot of support from a wing of the party that likes mean," said Katie Packer Gage, a Republican political consultant. "But that's not the whole party."
Packer Gage -- who was deputy campaign manager for Mitt Romney's presidential run in 2012 and whose firm does some work with Rubio -- said the senator's strategy has served Republicans well in the past.
"Rubio comes across as strong and resolute but always a gentleman. Mitt went far with that sort of demeanor," she said.
"I think you can be strong, tough and angry without being mean and classless."
- Everyone's second choice? -
But Rubio has struggled since an initial poll surge when he entered the race in April.
With six months until the voting starts in Iowa, he is polling at around seven percent in an extremely crowded field, well behind the front-running Trump.
In New Hampshire and South Carolina, which also vote in February, Rubio's figures are even worse, even though he enjoys some support from all wings of the party.
"Rubio remains very popular with the Republican electorate, but also remains unable to fully coalesce that popularity into a clear preference for him," consultancy firm TargetPoint said in a recent report.
"No one in the field has more room to grow than Rubio."
Packer Gage believes Rubio's broad appeal will ultimately pay dividends as the field narrows.
"Some candidates will struggle to raise money and drop out. Some will implode by making big mistakes and drop out. He's not likely to suffer from either. So he will be in it for a while."
"Everyone likes him. Except the most rabid anti-immigration types. He's a lot of people's second choice. As his exposure grows, his support will grow," she said.
But the electoral calendar shows just how narrow Rubio's window of opportunity might be.
Rubio will want to turn in a stellar performance at the next Republican debate on September 16 to propel him towards the end of the year, as he looks to raise money and build ground operations.
But the Nevada caucus at the end of February could be an inflection point for his campaign.
Rubio lived in Las Vegas as a child and was once a Mormon, an important voting bloc in the state.
Crucially for Rubio, Nevada is also culturally diverse and relatively youthful, so his optimistic message is one that might play well.
"When you start with states like New Hampshire and Iowa, where diversity is not a big priority -- generally all white states that are very rural -- it disadvantages candidates like Rubio," said Susan MacManus, a professor of politics at the University of South Florida.
A loss in Nevada would raise serious questions about Rubio's electability, while a win would catapult him into a string of southern states and the all-important Florida primary in March.
The Sunshine State could yet prove to be a knock-out battle royale with fellow Floridian and one-time mentor Jeb Bush.
As a large state, where expensive ad buys are more important than knocking on doors, Florida would seem to favor Bush, a former governor who has built up a considerable war chest.
But it could all come down to momentum.
"Florida is going to be really hard fought," said MacManus. "Whoever is doing best ahead of our primary will probably take Florida, because a lot of Republicans just want to win the White House."
Rubio will have to head home in March with significant juice, or else he risks just being a happy warrior who is everyone's second choice.