By Andrew Callus and Michel Rose
PARIS (Reuters) - For two years, disaffection with mainstream politics and disarray among her opponents have played to Marine Le Pen's agenda. But as the days tick down to election year in France, events may have started to dim her presidential prospects.
Since late November, the anti-immigrant, anti-globalization far-right National Front candidate's showing in opinion polls has slipped from about three out of 10 voters to one in four.
Her niece, the National Front member of parliament Marion Marechal-Le Pen, has re-opened old faultlines within the party.
And the policies and personalities of those emerging as Marine Le Pen's main opponents look set to make campaigning tricky for her in the months ahead.
The election rules are simple: the two candidates who win the most votes in an April first-round vote go through to a run-off round on May 7. By polling so well for so long, Le Pen's presence in that run-off had been seen as one of the few certainties in the race.
Not any longer.
"Le Pen not reaching round two would have been a wild bet even four months ago," the historian Justin Vaisse, director of the French foreign ministry's center of analysis, forecasting and strategy, tweeted this week.
"(That's) now credible -- even though many unknowns remain."
The choice on Nov. 27 of Francois Fillon as Le Pen's main conservative challenger in the Republicans party primary was among the first signs of trouble.
She put a brave face on the victory for the former prime minister, whose socially conservative views on abortion and gay marriage are attractive to many Le Pen supporters and helped land him the ticket ahead of the more centrist Alain Juppe.
She focused instead on Fillon's aggressive plans to slash public sector spending, calling it an attack on the French workers she and her second-in-command, Florian Philippot, have promised to protect.
But the impact in the polls was immediate.
A Harris Interactive survey after thE Nov. 27 primary put her on 24 percent of voting intentions, behind Fillon on 26 -- her lowest score in months, and one that has not improved since.
Then her niece Marechal-Le Pen twisted the knife, giving a Dec. 10 interview in the Journal de Dimanche Sunday newspaper.
The 27-year-old is ideologically much closer to her grandfather Jean-Marie, the party's now ostracized founder, than to her aunt Marine and party's modernizers led by Philippot.
From her Vaucluse constituency in the south of France, part of Marechal-Le Pen's line is a form of social conservatism not very different in voters' eyes to that of Fillon - with both expressing clear reservations on abortion and gay marriage.
"I am neither in a minority nor am I isolated," she told the newspaper, standing firm on proposals that the state should no longer pay for pregnancy terminations.
Just days earlier, party leader Marine Le Pen had made clear that she would not change France's abortion laws.
Marechal-Le Pen has also attacked Phillipot, who is gay, over his support for government-funded safe-sex posters depicting gay couples.
She called the poster campaign "an embarrassment for children and for homosexuals," and said: "The majority, in the FN, do not share that choice at all."
Fillon, a Catholic like Marechal-Le Pen, has said he would not change France's abortion laws, but has said he is personally opposed. He also wants to limit adoption rights of gay couples.
By stepping onto her traditionalist turf, Fillon leaves the Le Pen campaign more dependent on her pro-worker agenda, which includes a lowering of the retirement age, hiking minimum wages and preserving a generous welfare safety net.
In that area, she already faces fierce competition from the opposite end of the political spectrum -- Jean-Luc Melenchon of The Left Party, a hardliner and former Socialist who is running for president with the backing of the French Communist Party.
Melenchon is much more credible in this area than Le Pen, according to the political commentator Thomas Guenole.
"Le Pen is trapped between Francois Fillon and Jean-Luc Melenchon," said Guenole in an editorial for Le Figaro newspaper.
Melenchon was eliminated in the 2012 presidential contest but scored a sizeable 11 percent of first-round votes. He is doing at least that well or better in regular polls of voting intentions in 2017.
Another hurdle for Le Pen is the youthful Emmanuel Macron's announcement on Nov. 16 that he too is running for president, as an independent candidate who rejects the traditional politics of Left versus Right.
Opinion polls since then show the 38 year-old former economy minister consistently in third place. He is still well behind Le Pen and Fillon, but even before he launched officially, some of her worst scores were in scenarios where he stood against her.
Macron has yet to get down to deep policy specifics, and has targeted the center, but pollsters detect a brand of "populism-light", and say his independence and youth could blunt Le Pen's anti-establishment appeal.
"He takes (votes) from everyone," said Jean Chiche, a voting science researcher at Sciences-Po university in Paris.
Indeed, if current popularity levels were to convert into voting in April's first round, Le Pen would be out of the running at Macron's expense.
A Dec. 20 Odoxa poll of approval ratings made Macron the most popular politician in France on a score of 35 percent, ahead of Fillon on 31, and Le Pen on 27. Two days later, an Elabe poll made him political personality of the year on 48 percent, ahead of Fillon on 44, with Le Pen trailing at 24.
As for the ruling Socialists, should he win their January primaries as the polls predict, former Prime Minister Manuel Valls has a lot of ground to make up from the distant fifth place in polls, behind Fillon, Le Pen, Macron and Melenchon.
For now at least, his bid has little bearing on Le Pen's prospects.
But there is everything to play for in an election campaign that has already created many surprises.
Just over a month ago, Le Pen's likely main rivals included heavyweights such as former prime minister Juppe or former president Nicolas Sarkozy for the Republicans, and the incumbent Socialist President Francois Hollande.
After Fillon's surprise win and Hollande's shock decision to not seek re-election, three people who had dominated politics for decades were consigned to campaign history, shattering many of the certainties as to what will happen on May, 7 2017.
(Additional reporting by Gerard Bon; Editing by Brian Love and Mark John)