By John Irish and Daniel Flynn
PARIS, April 26 (Reuters) - Far-right leader Marine Le Pen, who has emerged as a kingmaker in France's presidential race, sought to wrest concessions from President Nicolas Sarkozy on Thursday by challenging him not to bar her party's way in parliamentary elections.
Both conservative Sarkozy and Socialist frontrunner Francois Hollande have courted Le Pen's voters since she took third place in Sunday's presidential first round with nearly one fifth of the vote, the National Front's best score.
Sarkozy, who trails his centre-left rival by 10 points in polls before a May 6 runoff, has made the most direct overtures to National Front supporters, saying he respected their vote for a party which has long been stigmatised.
Le Pen has promised to give her view on the second round at the National Front's traditional "Joan of Arc" May Day rally, and she urged Sarkozy to make his own position clearer concerning parliamentary polls in June.
Building on her record support, the National Front hopes to win its first seats in parliament since 1986, when an experiment with proportional representation gave it 35 deputies.
"In a runoff between the National Front and a Socialist, would the UMP and the president prefer to have one of my deputies or a Socialist elected?" Le Pen asked on RTL radio, referring to Sarkozy's centre-right Union for a Popular Movement party.
"I still don't have an answer to that question. I'm waiting," she said, when asked who she would endorse. "How I express myself will depend on the response."
Hollande, who said he understands voters' exasperation at high unemployment and a widening gap between rich and poor, has blamed Sarkozy for fostering the far right by aping its aggressive stance on immigration and national identity.
Sarkozy took up another Le Pen idea on Thursday, calling for a change in the law to allow policemen who open fire on suspects to be presumed to have acted in "legitimate self-defence" unless proven otherwise.
He made the call after hundreds of officers demonstrated in police cars on the central Champs-Elysees avenue after a colleague who shot dead a fugitive in a Paris suburb was placed under judicial investigation for suspected murder.
"I am not prejudging the justice system ... but as head of state I must defend the honour of our officers when it is called into question," he told frenzied supporters, who chanted "We are going to win!".
Opinion polls show supporters of Sarkozy's centre-right UMP party favour a deal with Le Pen, but the president has ruled out any agreement which would give the far-right ministerial positions or help them win seats at June's legislative election.
However, Sarkozy has stopped short of instructing UMP supporters to vote for any other party than the National Front in the second round of the legislative ballot to shut the far-right out of parliament, as politicians have done in the past.
Based on Sunday's results, the National Front could reach the second round in up to 345 of the 577 constituencies in the parliamentary election, splitting the right-wing vote.
MARKET JITTERS OVER HOLLANDE
Most polls show Hollande winning on May 6 by around 55 percent to Sarkozy's 45, thanks the backing of the vast majority of far-left votes and much of the political centre.
If elected, Hollande has pledged to slap higher taxes on the rich and large corporations, include growth measures to a German-inspired budget pact imposing austerity across Europe, and hire 60,000 new teachers.
The prospect of Hollande winning power has sent jitters through financial markets, even though the 57-year-old has insisted he is committed to balancing France's budget by 2017.
Sarkozy, whose flashy style has alienated many conservatives, needs around 80 percent of Le Pen voters to avoid defeat. But surveys after the April 22 first round found only 45 to 60 percent of her supporters would make the switch, down from 70 percent in 2007 when Sarkozy's tough immigration line helped him to the presidency.
Senior aides have suggested Le Pen is highly unlikely to endorse either candidate explicitly. The gravelly voiced smoker, who took over the party founded by her ex-paratrooper father Jean-Marie in January last year, has said she hopes to profit from an implosion of the mainstream right.
In an open letter to Hollande and Sarkozy on Thursday, Le Pen said she was not the "owner" of her first round votes and said it was unacceptable that her supporters were being branded as "xenophobes".
"I consider it my duty to defend the honour of my voters and insist that they are not scorned. I consider your attitude so far makes your quest for these votes particularly illegitimate," she said. "There is perhaps still time for you to understand."
Sarkozy, treading a fine line between alienating centrists and winning over Le Pen's vote, said he was certain that her supporters did not endorse extremist views and Le Pen herself had not displayed xenophobia.
"There may be xenophobia among certain leaders of the National Front, such as Jean Marie Le Pen, but I haven't heard it from her," he told France Inter radio.
An opinion poll showed two-thirds of Sarkozy supporters want him to break with past policy and strike an alliance with the National Front after Le Pen's 17.9 percent score on Sunday made her 6.4 million backers key to the presidential runoff.
In a setback to Sarkozy, centrist Francois Bayrou, who came fifth with 9.1 percent, accused the president of being "absurd and offensive" in comparing his voters with those of Le Pen.
In an open letter to both candidates on Wednesday, he called for more civil, clean and moderate politics, appearing to lean towards Hollande without explicitly endorsing him.
However, former centrist president Valery Giscard d'Estaing backed Sarkozy on Thursday, saying Hollande would not be able to change the tax-and-spend ways of the Socialist party which would expose France to "international speculation".
The former head of state said he believed the debate between the two candidates on May 2 could swing the final vote.
"A real debate can make or break things, even today," he told Le Parisien newspaper, referring to a debate he had with François Mitterrand in 1974, which narrowly swung the election is his favour.