By Andrew Osborn
LONDON (Reuters) - One is a brash right-wing billionaire celebrity tilting for the White House. The other a hitherto obscure hard-left lawmaker vying to lead Britain's defeated opposition Labour Party.
Yet Donald Trump, 69, and Jeremy Corbyn, 66, have more in common than meets the eye. Straight-talking populist insurgents, both are tapping into frustration on either side of the Atlantic with the prevailing orthodoxy of bland centrist politics.
Blending polarizing ideas with what admirers call refreshing authenticity, Trump and Corbyn have emerged improbably -- on opposite sides of the political spectrum and ocean -- as potential party leaders.
"Both of them are populists in the sense that they believe they are tribunes of the people who they contend have been sold down the river by the elite for too long," said Professor Tim Bale, one of Britain's leading political analysts.
"Their pitch is that they're going to come out and clean the stables and finally bring some common sense to government."
The success of Trump and Corbyn may not last and rivals cast them as unelectable. But their rise reflects a weariness among many political activists in the United States and Britain with what they see as bland centrist machine politics.
That, political analysts believe, speaks to a disconnect between members of the so-called Grand Old Party (GOP), and the British Labour Party, with the wider electorate.
"Both candidates are running for the leadership at points in history where their parties have significantly lost their way," said Dan Hodges, a former Labour adviser who follows politics on both sides of the Atlantic.
"The Republicans are coming off the back of two successive presidential election defeats and Labour lost big in May. You've got two parties who have forgotten how to win."
CHALK AND CHEESE
On the face of it, the two men could not be more different in style or substance.
Trump, a wealthy real estate magnate, is a household name, a self-styled patriotic man of the right, and a capitalist standard-bearer.
He opposes abortion and same-sex marriage, deems climate change a fiction, and has declared China "an enemy".
Corbyn, a veteran Labour lawmaker noted for his frugality, was little known until recently, even inside his own country.
Quietly-spoken, he opposes nuclear weapons, has called Hamas and Hizbollah -- groups designated as "terrorist organizations" by the United States -- "friends," and admires Karl Marx.
The two candidates do have something big in common however: they have both spooked political elites by unexpectedly dominating the race to control their parties, destabilizing the organizations they seek to lead.
Both are banking on the demise of centrist politics shaped by "spin doctor" consultants who test policy ideas, as if they were cat food or detergent, on focus groups of voters.
President Barack Obama and Prime Minister David Cameron may have won two terms in office apiece by steering their parties broadly towards the center ground.
But Trump and Corbyn, channeling distinctive strands of opinion in their parties, are betting that voters yearn for a return to a left-right battle of ideas.
That has gone down well with influential parts of their parties' bases. But some politicians and analysts believe it is a recipe for electoral disaster.
"Why have both of these parties, which were mainstream players in their national democratic processes, suddenly gone mad and swung toward the most extreme, electorally unviable versions of themselves?" Janet Daley, a U.S.-born political commentator, wrote in the Daily Telegraph newspaper.
"And even if these would-be leaders do not win through to the top of their respective parties, why are they being allowed (encouraged?) to do the kind of damage that will be hard to live down?"
Few predicted either man would mount a serious challenge to lead their party.
Critics have long derided Trump as a lightweight buffoon who would be laughed out of town. Instead, the "Trumpster", as he is known in parts of the media, has emerged at or near the head of a 16-candidate primary to choose the Republican Party's presidential nominee ahead of next year's White House election.
A July Washington Post and ABC poll credited Trump with 24 percent support among Republican and Republican-leaning voters.
Corbyn too has confounded the skeptics. He struggled even to make it onto the initial slate of four candidates wanting to lead Britain's Labour Party, in disarray after suffering its worst defeat in almost three decades in May.
Yet with little over a month to go before the centre-left party chooses its new leader, Corbyn is the favorite among some bookies and pollsters.
Neither man nods to convention.
Trump, who Forbes magazine estimates is worth $4bn, has offended Mexicans, saying illegal immigrants from the Latin American country are rapists who bring drugs and crime to the United States.
His comments have alienated many potential voters, a potential problem for someone who would need to win Hispanic votes to have any chance of taking the White House.
Corbyn too is seeking to defy conventional wisdom.
Senior Labour politicians concluded after May's election defeat that their party had tanked because its then leader, Ed Miliband, had lurched to the left.
Corbyn thinks it didn't tack left enough however and wants to push it further from what triple-election winner and former leader Tony Blair regards as the electoral sweet spot.
"We can win an awful lot of people into the political spectrum, by offering something that is ... radical," Corbyn said last month, when asked about Blair's theory of how to win elections.
Such talk fires up activists. But it goes down less well with voters. Some Americans view Trump as an ill-informed right-wing xenophobe, and some Britons regard Corbyn as a hard-left ideologue who would wreck a resurgent British economy.
"They will both lose," predicted Hodges. "The Corbyn insurgency has peaked, and despite the hype Trump has never been a really serious candidate for the Republicans, something that will become more evident with the passage of time."