Valérie Trierweiler posted an apparently innocent message on Twitter wishing a relatively unknown politician good luck in his bid to win a seat in the National Assembly in Sunday's second round parliamentary vote.
But the tweet stunned France as it was obvious that the real target was Ségolène Royal, the woman who shared President Francois Hollande's life for three decades and is the mother of their four children.
Royal, who failed in 2007 to get herself elected president, is standing against Olivier Falorni, a Socialist dissident, for a parliamentary seat for the western town of La Rochelle.
Hollande has publicly thrown his weight behind Royal, writing this week that she is "the only candidate of the presidential majority who can be assured of my support" in the constituency.
Socialist Party leader Martine Aubry travelled Tuesday to La Rochelle to show her backing for Royal, declaring that she had the unwavering support of the entire party, which is tipped to win a majority of seats in Sunday's vote.
But just as she made that ringing endorsement, Trierweiler delivered her blow on the micro-blogging site: "Good luck to Olivier Falorni who has done nothing worthy of blame, who has fought alongside the people of La Rochelle for so many years with selfless commitment."
Royal, who is hoping to become parliamentary speaker if elected, declined to react directly to the taunt.
"While there are divisions, while there are personal attacks and gossip, the people are scornful because they are looking for something else from politics," she told a national radio station.
"All my spirit, all my energy, all my thoughts are for the voters" of La Rochelle, she added.
Aubry, when asked about the tweet, said that all that mattered was that Hollande had backed Royal.
Reports of rivalry between Trierweiler and Royal had led to speculation, which Trierweiler denies, that the current first lady had Royal airbrushed out of a film screened to Socialist faithful at Hollande's January campaign launch.
Hollande stood loyally by Royal as she battled Nicolas Sarkozy for the presidency in the 2007 race but he had reportedly been in a relationship since 2005 with Trierweiler, a twice-divorced 47-year-old journalist and mother of three.
The tweet by Trierweiler, who has kept her job at Paris Match magazine and has not married Hollande, caused dismay in the Socialist party and sparked derision from right-wing opponents.
Eric Ciotti, from Sarkozy's UMP party, declared that "vaudeville has come to the Elysee" presidential palace, while another UMP politician said "it's 'Dallas' at the Elysee", referring to the steamy US soap opera.
Socialist deputy Jean-Louis Bianco said he found Trierweiler's tweet "purely and simply disgraceful".
Trierweiler had a previous catty outing on Twitter during Hollande's presidential campaign, when she lashed out at Paris Match after finding herself and her boyfriend on the front page.
Their photo was accompanied by the headline: "Francois Hollande's charming asset. The story of how their love was born."
Trierweiler, who supported Hollande on the campaign trail and had an office at his headquarters, tweeted her fury at not being informed by her employer that she was their top story for the week.
She sarcastically congratulated the magazine, in a second, even angrier tweet, for reducing her to a trophy companion.