President Nicolas Sarkozy's threat to raise unilateral barriers to trade and migration unless the European Union toughens its stance has amplified a populist tone in France's caustic election campaign but could rebound against him.

Both Sarkozy and his Socialist challenger, Francois Hollande, are trying to win back voters tempted by the political extremes with simplistic proposals that experts doubt they could or would implement in practice.

Hollande has proposed a 75 percent top income tax rate for millionaires aimed at shoring up his left-wing base.

But the conservative president may have more to lose from threats, spelled out in a speech to a mass campaign rally on Sunday, to pull France out of Europe's open-border zone of passport-free travel and apply protectionist trade measures.

Sarkozy's message was aimed chiefly at supporters of far-right anti-immigration candidate Marine Le Pen, who stands third in opinion polls, but it conflicts with his efforts to project himself as a European statesman.

Scapegoating Brussels over immigration and globalisation could alienate centrist voters and give the outside world the impression that France is turning inward again, as it did when voters rejected a draft EU constitution in a 2005 referendum.

It sends off a very negative signal, Thomas Klau at the European Council on Foreign Relations said of Sarkozy's gambit.

He is recycling a political strategy he's used successfully in the past. The fact that this time he's a president in office makes this strategy more problematic, particularly when it comes to questioning the functioning of important EU policies.

The conservative leader's vow to defy Brussels if necessary to defend French interests follows campaign pledges to halve the flow of immigrants into France, impose minimum company taxes and enforce the labelling of ritually slaughtered halal meat.

Sarkozy's Europeanism had been one of his better traits but now he's shed his skin like a snake and donned another, at least for the length of the campaign, Harvard academic Arthur Goldhammer wrote in his French Politics blog. The climate deteriorates by the day.

As Sarkozy shifts to the right in a battle to catch Hollande in opinion polls, the Socialist candidate has veered to the left to claw back voters from firebrand leftist Jean-Luc Melenchon, credited with up to 10 percent support.

Hollande also wants to incentivise local production, halt the relocation of companies abroad and link wage rises to economic growth. But aides say he would be a more centrist and pragmatic leader than his leftist campaign stance would suggest.

He describes himself as more pink than red and says his 75 percent tax rate proposal is symbolic and would affect very few people.

While Hollande's manifesto omits any mention of structural economic reforms, a close aide told Reuters he fully intends to work with trade unions on labour market reforms and investment policies to try to repair France's competitive disadvantage.

Hardline Socialist Arnaud Montebourg said that Sarkozy's ultimatum to Brussels smacked of electoral panic. Fellow left-winger Alexis Corbiere called it a headlong chase after Le Pen.

Greens presidential candidate Eva Joly said that to threaten exiting the Schengen zone was akin to stabbing Europe in the back and sacrificing it on the altar of Sarkozy's campaign.

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While polls show many voters have been turned off by an election race more focused on personalities than on core issues, 61 percent support Hollande's top tax rate. Immigration curbs and protectionist policies also tend to be popular.

Blighted in his re-election bid by rock-bottom popularity ratings and public anger with years of economic crisis, Sarkozy gained a few points in polls with an energetic campaign launch in mid-February, but has since slipped back.

Addressing some 30,000 supporters on Sunday, Sarkozy said he would suspend France's membership of the 25-nation Schengen zone and start favouring French companies for government contracts unless he saw progress on EU border controls and some kind of Buy European legislation within a year.

EU diplomats say Sarkozy is merely dressing up in tougher election language existing French positions that are already driving debate in Brussels. The message is extreme but the content is not new, said one European diplomat in Brussels.

Hannes Swoboda, Austrian floor leader of the Socialist group in the European Parliament, said Sarkozy was betraying his fear of losing the April-May election. Still, such comments from a governing president are scandalous even during an election campaign, he said.

European Commission spokeswoman Pia Ahrenkilde-Hansen declined to engage with Sarkozy's comments, saying: We do not comment on national election campaign speeches.

Even Eurosceptical analysts questioned whether Sarkozy could carry out his threat.

It's very unlikely that he would go ahead and suspend Schengen membership because that would mean major disruption, said Mats Persson, director of the London-based Eurosceptical think-tank Open Europe.

Both of his promises would be difficult to deliver on.

The Schengen zone is so central to the EU's core principles that pulling out would raise serious legal and practical issues. France would have to re-introduce border posts with friendly neighbours such as Belgium, Germany, Spain and Italy that were dismantled almost two decades ago.

It would also anger the many French people who enjoy passport-free European travel.

The fact that Brussels is already looking at ways to toughen up the EU's external borders, after prompting by Paris and Rome, means that if re-elected, Sarkozy could well say he has seen enough progress not to have to make good on his threat.

The door is open for him to say: Well, my demand has been met. So I think the threat is largely rhetorical, said Klau.

Eurasia analyst Antonio Barroso said imposing unilateral trade protectionist measures would violate EU treaties and could land Paris in court. (It) would entail a violation of treaties and would bring about infringement procedures by the Commission, he said.

Even centre-right politicians said Sarkozy's proposal was a political gesture that would never be put into practice.

You'd have to be crazy to think we are going to rehire customs agents and re-establish frontier posts in France, said centrist presidential rival Francois Bayrou. Everyone knows that won't happen. We will not take that big a step backwards.

Rather than rattle outsiders, Sarkozy's populist push is a gamble which may not pay off for his re-election bid.

It's a high-risk strategy which he wouldn't have chosen if he had better poll ratings. By sounding excessively shrill, he might actually make Hollande look more presidential, said Klau.

(Additional reporting by Daniel Flynn in Paris; Justyna Pawlak in Brussels and Michael Shields in Vienna; Editing by Paul Taylor)