* Fillon says scandal a Socialist plot to halt his candidacy
* Socialists say accusation "not acceptable"
* Polls show rising support for Le Pen, Macron
* Presidential election to be held in April, May

Conservative presidential candidate Francois Fillon denounced a scandal engulfing his campaign as a plot by France's ruling Socialists on Wednesday, asking backers to stay loyal despite an opinion poll showing he had lost his status as favourite.
The poll by Elabe pointed to rising support for far-right National Front leader Marine Le Pen and made centrist Emmanuel Macron most likely to win the presidency, with voters appearing to desert Fillon in large numbers.
"We are facing an institutional coup d'etat," Fillon told The Republicans party lawmakers at a meeting, according to one who was present. "This operation does not come from within our own camp, this affair is coming from those in power".
The lawmaker, who wanted to remain anonymous, said Fillon asked his fellow conservatives to "hold on for 15 days" - long enough for an official inquiry into the affair, which opened last week, to run its course.
"Nobody will stop Francois Fillon and the Right from being present in the first round of the presidential election and the second round of the presidential election," Fillon's spokesman Thierry Solere said.
Responding for the Socialists, government spokesman Stephane Le Foll said the accusation was "not acceptable".
It was a report in The Canard Enchaine satirical newspaper last week that kicked off the scandal. The newspaper has reported that Fillon's British wife Penelope and some of the couple's children were paid over €1million in all, most of it taxpayer-funded and mostly as assistants to Fillon and a successor, with very little evidence of real work.
Fillon has said the work was genuine and that he would step down should the preliminary inquiry lead to a full judicial investigation.
The couple and some of their associates have since been interviewed by police.
Some members of the candidate's party were this week considering how to deliver a campaign without him. It was unclear how a new candidate could emerge with less than three months to go before the election.
Wednesday's poll by Elabe for Les Echos newspaper was conducted on January 30 and 31, a few days after the story broke.
Voting intentions for Macron were unchanged or up 1 point to 22-23%, depending on different candidate scenarios. Le Pen's first round score was up by 3 points to 26-27%.
Fillon's score fell by 5-6 points to 19-20%.
Both scenarios would eliminate Fillon from the May 7 second round, a contest between the two top scorers in the April 23 first round.
Market impact
Elabe predicted Macron would beat Le Pen in the second round with 65% of the vote. Should Fillon make the second round, he too would beat Le Pen, but by a lower margin, at 59%, Elabe said.
The poll, published on Wednesday and covering about 1,000 people, is the second one to be conducted since the scandal engulfed Fillon's campaign. It confirms a slide in the vote for Fillon that was noted by a survey published on Sunday.
The revelations against Fillon are particularly damaging because of the squeaky clean image that won him his party ticket in last November's primaries, Yves-Marie Cann, head of political studies at Elabe told Reuters.
"These accusations are hitting at the load-bearing wall of his popularity," Cann said.
Fillon is bleeding supporters from all sides, Cann said: hard-right voters who used to back ex-president Nicolas Sarkozy and are now tempted by Le Pen and centre-right voters are now leaning towards Macron.
The Fillon scandal has heightened investor concerns amid fears that Le Pen could win and take France out of the euro and the European Union.
On Wednesday, the gap between French and Belgian government borrowing costs hit its widest since at least April 2008.
"The France spread to Belgium is the gauge we use for political risk, and that has widened further after an adviser to Le Pen fleshed out their Frexit plans," said ING strategist Martin van Vliet. "And with Fillon under the microscope as well, France is definitely underperforming."