BRUSSELS (Reuters) - It may sound counter-intuitive after the Euroskeptic Finns Party grabbed second place in Finland's general election, but a surge by anti-establishment protest groups sweeping Europe may be peaking.
With the exception of Greece, where a five-year depression propelled far left, anti-bailout Syriza to victory in January, radicals are unlikely to win power outright in any other European Union state this year, opinion polls suggest.
A nascent economic recovery, falling unemployment in many countries, cheaper fuel and low interest rates should help mainstream center-right and center-left parties regain some lost ground in time for national elections.
In Britain, anti-EU insurgents who gave mainstream parties a kicking in last year's European Parliament elections are struggling with the full glare of electoral scrutiny. The UK Independence Party is on course to win 14 percent of the vote on May 7 but will capture a handful of parliamentary seats at best.
In other countries such as in France, populist parties seem to have hit a ceiling, barring some extraordinary crisis.
"Voters are more inclined to indulge themselves and cast protest votes in secondary elections where the stakes are less decisive," said Christian Lequesne of the Sciences-Po institute in Paris.
Fringe groups have already won a victory of sorts by dragging mainstream parties onto their ground, particularly on immigration, European integration and euro zone bailouts.
The growth of right-wing protest parties in Germany, the Netherlands and Finland has already prompted governments to toughen their line on any further financial aid to Greece.
German officials say one reason Finance Minister Wolfgang Schaeuble has struck such a gruff posture with Athens is to ward off a domestic challenge from the Euroskeptic Alternative for Germany, which has won seats in the European Parliament and state assemblies but not yet in Berlin.
In Britain and France, mainstream conservatives have played up the fight against illegal immigration, so-called "benefit tourism" within Europe and radical Islam, partly to try to wrest those issues away from Nigel Farage's UKIP and Marine Le Pen's National Front.
But with none outside Greece likely to win power alone, the radicals face a choice of joining coalition governments and moderating their demands -- which can cost them support -- or eschewing power to stay shouting from the outside.
"Those kinds of parties are probably better off staying outside and trying to exert influence," said Tim Haughton, an expert on European politics at Birmingham University in England.
Janis Emmanouilidis, director of studies at the European Policy Centre in Brussels, is less convinced that the populist wave has peaked, arguing that events could give it new vigor.
"A Greek default, if it happens, would definitely be a game-changer in a number of countries. The populists would say 'I told you so'," he said.
Others argue that if Greece implodes, European voters will think twice about voting for parties like Syriza.
Mass drownings of illegal migrants trying to reach Europe in the southern Mediterranean could also give right-wing populists fresh wind by polarizing debate about migration, notably over sharing the resettlement of migrants across the EU.
More attacks by Islamist militants in Europe like January's Charlie Hebdo shootings in Paris could also fuel such political groups.
Even before the 2008 financial crisis, mainstream center-right and center-left parties were losing voters and members as traditional class-based affiliations fade in an era of individualism, opening the way for upstarts amplified by social media.
A splintering political spectrum makes stable parliamentary majorities increasingly hard to find.
In Britain, where a center-right coalition established in 2010 broke with a tradition of alternating single-party rule since World War Two, pundits see little chance of either Prime Minister David Cameron's Conservatives or David Miliband's opposition Labour Party winning an outright majority.
Regional parties such as the Scottish Nationalists and the Northern Ireland Democratic Unionists may end up as kingmakers and some experts forecast an unstable minority government possibly triggering another election soon.
Finland's election on Sunday was a test case for the fortunes of protest parties, which face a dilemma when they have to choose whether to compromise with mainstream politicians.
Timo Soini's Finns party may enter government for the first time as junior partner to election winner Juha Sipila's Centre party, having softened its anti-euro rhetoric since bursting onto the scene four years ago.
The Finns actually lost two percentage points of voting share and won one fewer seat than in 2011.
How the charismatic Soini balances cabinet responsibility with railing against aid for Greece, especially if Athens needs a third bailout package, will be a big challenge.
Fed up with the political wilderness, the 52-year-old feels he made a mistake by turning down a cabinet seat four years ago.
"It is important that the party moves to a next stage," Soini told Reuters. "The power is in the government."
Another test looms in Spain later this year when the ruling center-right People's Party and opposition center-left Socialists, both tarnished by mass unemployment, austerity and corruption scandals, face a two-pronged insurgency from left-wing Podemos (We Can) and the centrist Ciudadanos (Citizens).
Podemos, led by pony-tailed political scientist Pablo Iglesias, is close to Greece's Syriza and was born out of street protests by young unemployed "indignados" (the angry). It briefly overtook the two mainstream parties in opinion polls this year after surging into the European Parliament.
But it came a distant third in the key southern Andalucia regional election last month and polls show its support slipping from the upper 20 percent range. Ciudadanos, led by 35-year-old Albert Rivera, is gaining ground by preaching moderation and clean government.
In France, Marine Le Pen, whose National Front topped the poll in the European elections, has worked hard to "detoxify" the movement's extreme-right image. She has just won a power struggle with her father, party founder Jean-Marie Le Pen, forcing him to bow out of regional elections this year after he repeated comments belittling the Nazi Holocaust.
But Lequesne said she was unlikely to gain critical mass in her 2017 presidential bid as long as she advocates leaving the euro and the EU and closing France's borders to imports, scaring middle class voters worried for their savings and property.
Recent history shows that novice populists can burn out fast if they rush into government as junior partners without being able to transform policy.
Hard-right anti-immigrant parties founded by the late Joerg Haider in Austria and the late Pim Fortuyn in the Netherlands fell apart and came off worst after joining ruling coalitions respectively in 1999 and 2002.
Yet in both countries, successor groups with a similar anti-immigration, anti-Islam and Euroskeptic agenda have since risen in opposition, forcing the established parties to change their discourse on those issues.
Editing by Mike Peacock