The ruling Socialist party and the centre-right Union for a Popular Movement (UMP), the party of former president Nicolas Sarkozy, face humiliating reverses in municipal and European elections next year if the FN can sustain its current standing in the eyes of an electorate thoroughly fed-up with record unemployment, rising taxes and a perceived increase in crime and insecurity.
A poll published last week suggested the FN could emerge as the best-supported party in the European elections with 24 percent of those asked declaring themselves ready to back the party led by Marine Le Pen, the daughter of FN founder Jean-Marie Le Pen.
That shock survey was followed on Sunday by a spectacular victory for the FN in a local by-election in Brignoles, where FN candidate Laurent Lopez claimed 53.9 percent of the vote in a run-off against the UMP candidate.
"The left and the mainstream right are blaming each other for what is happening, but the reality is they've both been knocked sideways," said Nonna Mayer, the Research Director at the National Research Centre CNRS. "Neither of them know what to do."
There were particular, local factors in Brignoles which influenced the outcome of a vote being held for the third time, the result having been judged too close to stand on two previous occasions, when the run-off was between the FN and the Communist Party.
The southeastern town has struggled with high unemployment since the closure of local aluminium mines in the 1990s and the gloomy economic backdrop has exacerbated tensions between established residents and a large community of North African immigrants, creating fertile ground for the FN in a region where it traditionally does better than elsewhere in France.
But the scale of the victory for Lopez was nevertheless widely interpreted as an indicator of how the FN is capitalising on current voter concerns to appeal to a broader slice of the electorate than ever before.
A smartly-dressed former businessman, Lopez, 48, is the perfect embodiment of the more voter-friendly image that the FN has projected since Marine Le Pen took over from her controversial father at the head of the party in 2011.
Harder to treat as a pariah party
The pledges to end net immigration, most controversially by ripping up family reunion rules, and to begin moves to pull France out of the European Union, remain in place.
But the FN is now far from being a one-issue party, notably making its voice heard on education to the extent that it has recently been able to announce the creation of a network of supporters amongst teachers - an unthinkable development a decade ago.
Marine Le Pen, has worked hard to dispel the image of the party as fundamentally racist.
She has expelled activists who make bigoted public statements and the FN lists in next year's municipal elections will include a handful of ethnic minority candidates.
All of which is making it harder for the mainstream parties to prevent the FN from winning more than a handful of elected posts by treating it as a pariah party and urging their voters to cast their ballots tactically to keep them out of power, an approach referred to as the "Republican Front".
"I think we can safely say the Republican Front is now dead," Marine Le Pen declared after Sunday's triumph in Brignoles, and her opponents acknowledged that she was probably right.
"It just doesn't work, voters don't like being told who to vote for," said Thierry Mandon, the spokesman for the Socialists' parliamentary party.
Despite the current disarray of the mainstream parties, the CNRS's Mayer still believes the recent upturn in the fortunes of the FN remains primarily a protest phenomenon.
"The change of leadership has given the party a slightly slicker image, but for most French people, it remains an extreme right party founded on racist ideas which is incapable of governing," Mayer told AFP.
"Marine Le Pen does have a better image than her father. Unlike him she is not associated with the legacy of the second world war and anti-semitism.
"Local factors played heavily in their favour in Brignoles. On a national level there is enormous disappointment after a year of the Socialists back in power, and the issues of crime and insecurity are beginning to weigh heavily in the national debate, which plays in the FN's favour.
"But don't forget that, in the municipals, the FN will have candidates in one town in six at most. It is not as strong as it pretends to be."