PARIS: French far-right leader Marine Le Pen is staging a comeback in opinion polls less than 100 days before the presidential election, riding a wave of economic discontent that has endangered President Nicolas Sarkozy's re-election chances.

As rising unemployment and the loss of France's triple-A credit rating tarnish Sarkozy's presidential record, pollsters say that Le Pen's brand of anti-European rhetoric is gaining ground with a broad swathe of unhappy voters.

On Thursday, a daily survey of voting intentions by pollster Ifop gave her 21 percent support in the first round of a two-stage election scheduled for April and May, just two percentage points behind the conservative head of state.

Analysts say the poll fits with a broader trend of growing support for Le Pen and her ideas that weakens both Sarkozy and his Socialist rival Francois Hollande, who leads the field.

A first-round exit for an incumbent president would be a first in the history of France's Fifth Republic, founded in 1958.

"We cannot say today with certainty that either Francois Hollande or Nicolas Sarkozy will be qualified in the first round," said Jean-Daniel Levy, an analyst with polling agency Harris Interactive.

"What is most surprising to us is that we have never seen such an elevated level of support for a far-right candidate three months before a presidential election," he added.

Le Pen, who holds a strong third position behind Hollande and Sarkozy, has made a push for mainstream respectability in the year since she took over leadership of the National Front from her father, Jean-Marie.

Shifting away from its narrow focus on immigration, Le Pen has rebranded the National Front as a protector of French sovereignty, ready to abandon the euro and shut French borders to foreign competition.

A spate of industrial shutdowns has bolstered support for her idea of "intelligent protectionism" among disenchanted working class voters. Troubled Swiss refiner Petroplus became the latest to raise the spectre of a closure, announcing on Friday it was looking to sell its refinery in Petit-Couronne, northern France.

"It's obviously the ultra-liberal policies defended by all presidential candidates, aside from myself, that is behind the closure of all these factories," Le Pen told RMC radio, referring to both mainstream candidates.

Ifop pollster Jerome Fourquet said Le Pen's comeback after a poll dip in late 2011 reflected disappointment with Sarkozy's record on employment, with jobless claims at a 12-year high, and anxiety over the downgrade of French debt to AA+ by Standard & Poor's last Friday.

"Bad news on the economic front clearly weakens the outgoing president, who had set a target of bringing unemployment down to five percent by the end of his term," he said. "All of that gives Marine Le Pen new arguments to criticise the president's mistakes."

Sarkozy, who has yet to announce his candidacy officially, used a speech in Lyon on Thursday to launch an indirect attack on Le Pen's ideas, saying a withdrawal from the euro would kill jobs and sink the French economy.

Boosting French competitiveness, notably by lowering labour costs, was "the only way", he said.