PARIS (Reuters) - A strong showing by Marine Le Pen's National Front in a by-election run-off in eastern France is the latest sign of the rise of the anti-immigrant party, with voters ignoring calls by mainstream politicians to snub her candidate.
President Francois Hollande's ruling Socialists clung on to Le Doubs parliamentary seat with 51.4 percent of the vote, barely 800 votes ahead of the National Front candidate. The main conservative contender was knocked out in the first round.
The far-right FN made unprecedented gains in municipal and European elections last year and surveys show Le Pen is a viable presidential challenger in 2017. Prime Minister Manuel Valls has warned her party stood "at the gates of power".
The FN's Le Doubs candidate Sophie Montel managed in the run-off to increase her first-round score by more than 6,000 votes despite appeals by the conservative UMP for supporters to abstain. The result underlined the FN's potential to secure gains in county and regional elections in March and December.
"What's clear is that the National Front is stealing lots of voters both from the UMP and the Socialists regardless of what party leaders tell them," said Jerome Fourquet, analyst for pollster IFOP.
Just over 30,000 votes were cast in the mid-term by-election.
Hollande has seen his popularity ratings double from record lows in the wake of last month's Islamist attacks in Paris.
For years France's main parties have easily blocked far-right candidates in election run-offs by urging voters to support whichever candidate is best placed to beat the FN.
When FN founder Jean-Marie Le Pen, Marine's father, reached the final round of the 2002 presidential election, left-wing voters ostentatiously held their noses in distaste as they cast their ballots for Jacques Chirac. The conservative incumbent won a landslide 82 percent.
The so-called "Republican Front" worked again in 2009, when a leftist candidate set to lose a local election to the FN in the northern town of Henin-Beaumont saw his vote tally triple between the two rounds after a call from mainstream parties.
But the anti-FN front is weakening as Marine Le Pen strives to make her anti-EU, anti-immigration party more appealing to mainstream voters and as the policy is questioned among some UMP figures, including its chairman, ex-president Nicolas Sarkozy.
"There is a real danger, especially when the blockade is not resilient enough to hold back a political group that does not espouse our democracy's core values," Valls told French radio.
Le Pen on Sunday declared her party the "real winner" of the Doubs election and her niece Marion Marechal-Le Pen, a deputy in the national parliament, said she "couldn't care less" whether other parties sought to block its rise.
Opinion polls show the National Front could take as many as 28 percent of votes in round one of March's elections to France's departments, the rough equivalent of counties.
With potentially dozens of FN candidates competing in run-off rounds in March, mainstream parties will in many cases face a choice to work together to block the FN or stand back and risk letting Le Pen's party win.
Both parties face potential losses but the choice has proved harder for the UMP. Sarkozy at first gave no instruction to voters and then issued a tentative call for them to avoid voting for the FN. Finally he was over-ruled by his party's executive committee who issued a statement urging voters to vote neither FN nor Socialist, in effect suggesting they abstain.
Pollsters warned while the FN was unlikely to win many departments in March due to unfavorable electoral arithmetic, even a few victories could destabilize mainstream parties whose power is rooted in local institutions.
One radical idea to keep the FN from winning power is increasingly floated by pollsters and political scientists -- disband the UMP and Socialist parties altogether and form a broad centrist group designed to win power.
Editing by Mark John and Janet Lawrence