The vote was the more striking in that the country's ruling Socialist Party didn't field a candidate and urged its supporters to vote for the conservative candidate to block the FN's path.
In the past, such a common front against the FN has managed to overwhelm its vote. That ploy didn't work this time, even though the turnout was unusually high for a by-election as almost half of registered voters casting ballots.
Last week, a French opinion poll on voting intentions for the European Parliament elections in May found the FN in the lead, ahead of both the ruling Socialist Party and the main conservative opposition, the Union for a Popular Majority. The poll, in the respected weekly Le Nouvel Observateur, gave the FN 24 percent, against 22 percent for the conservatives and 19 percent for the Socialists.
"The FN is changing in nature. Its place is no longer at the margins of the political game but at the center," the magazine commented in an editorial.
In Holland, the anti-immigrant Party for Freedom, known by its Dutch initials PVV and led by Geert Wilders, also leads in opinion polls, as the coalition centrist government grapples with an austerity program required by its partners in the European Union.
In Austria last month, the anti-immigrant Austrian Freedom Party won more than 20 percent of the vote and almost stopped the ruling grand coalition of the two main center-right and center-left parties from getting a majority of the votes.
Across Europe, a tide seems to be running. But identifying the precise nature of that tide is harder than it looks. In France and Holland, and in Norway where the conservatives rose to power with tough promises to curb immigration, the tide seems to be against immigration. But the outpouring of sympathy last week when more than 300 would-be illegal immigrants drowned on a boat that sank en route from Libya to Italy suggests that Europe isn't exactly being swept by racism.
In other countries in Europe, the more potent issue seems to be frustration with the European Union and its institutions, widely blamed for the policies of austerity which have sent unemployment sky-rocketing to more than 20 percent in several countries.
In Britain the U.K. Independence Party, which favors pulling out of the European Union altogether, is rising in the polls and claims to be the country's legitimate third party. Britain's ruling conservatives fear the prospect of UKIP taking so many of the votes they lose the next elections.
Meanwhile, another euroskeptic party, Alternative for Germany, just missed out on a place in the Bundestag in last month's general election. It came from nowhere to fall just short of the threshold of 5 percent the national vote needed to enter Parliament.
But is the rise in the vote of these once-fringe groups the result of immigration, or of skepticism about Europe, or is it simply a protest vote against hard times and economic stagnation? Most likely, it is a combination of the three, combined with distrust of conventional politicians. Whatever the components of their appeal, it makes it difficult to dismiss these parties as "extreme right" or racist or even neo-Nazi. It is more complex than that.
Austria's Freedom Party is drawing mass support from working-class, blue-collar voters, University of Vienna political scientist Sylvia Kritzinger says, "Because of the policies they put forward, like immigration reform, anti-European integration, anti-corruption in the political system."
In France, the FN appears to be losing its pariah status and appealing to disillusioned socialist and conservative voters with promises to crack down on crime and illegal immigration and to leave the euro currency. Under its new leader, Marine Le Pen, a lawyer and mother of three, the FN's image has both broadened and softened since the days when it was founded and led by her father, Jean-Marie Le Pen, who was easily dismissed as an extremist for calling the Holocaust "a footnote of history."
In last year's presidential election, when she won 17 percent of the vote, almost 1-in-5 women and nearly a quarter of those aged 18-25 voted for her.
The real tide that is running is the steady erosion of the traditional two-party system of left and right that harks back to the days of Karl Marx when a party of capital faced a party of labor. That two-party system no longer reflects social reality, when the labor unions are weak and there is no longer a mass industrial working class. Moreover capital tends to mean pension funds rather than rich factory owners in top hats.
The growth of new parties like the Greens and euroskeptics, or regional parties in Scotland and Catalonia, or single issue groups like the briefly popular Pirates in Germany, suggests that loyalties are weakening to the old two-party system. The new influence of the Tea Party among Republicans suggests that something similar seems to be developing in the United States.