Only days before tomorrow's deciding round of parliamentary elections, Trierweiler tweeted her support to a party dissident who is running against Royal in a western constituency and has a good chance of beating her.
"Good luck to Olivier Falorni, who has proved himself worthy, who has fought alongside the people of La Rochelle for so many years with selfless commitment."
Outwardly saccharine and innocent, her tweet is being seen by some as a poison-tipped arrow fired by a woman unhinged by jealousy.
She sent it just hours after Hollande surrendered to pleas from his colleagues in the Socialist Party and publicly endorsed Royal.
"I am completely gobsmacked," a presidential adviser told journalists. "I was expecting government crises, not marital ones. It's amazing."
The high point in Royal's career was in 2007 when she ran for the presidency as the Socialists' champion, in a campaign led by the self-effacing Hollande, the party chief at the time.
She was crushed by the conservative candidate, Nicolas Sarkozy, but kept herself in the national eye thanks to her other job, as head of the Poitou-Charentes regional council in western France.
She is now standing against Falorni for a seat in the western town of La Rochelle in a bid to secure her return to national politics.
Paris is abuzz with tales of how Royal brokered a deal that Hollande would appoint her speaker of the National Assembly, making her the country's fourth highest dignitary, in return for her support during his presidential campaign.
Falorni is standing as an independent after being expelled from the Socialist Party after 25 years for refusing to make way for Royal.
"It is an improbable movie script, mixing political rivalries and private jealousies, power plays and intimate secrets and all at the highest level of the state," wrote the left-leaning daily Liberation.
Hollande campaigned for president as Mr Normal, a down-to-earth safe pair of hands after the turmoil of Sarkozy.
The balding and myopic Hollande told voters he would remain above party politics and put them as his priority. He also promised an end to the domestic distractions that marked his predecessor's first year in office.
In 2007, only five months after Sarkozy took office, his second wife Cecilia fled the Elysee into the arms of her lover; four months later Sarkozy married former supermodel Carla Bruni. In September 2008, dressed in a strapless red ball gown, Bruni posed on the roof of the Elysee Palace for the American magazine Vanity Fair.
When Trierweiler accompanied Hollande into the Presidential Palace just five weeks ago she promised something more refined, more in keeping with the office of Premiere Dame. After all, the Sorbonne-educated Trierweiler told reporters, after two decades as a political reporter for Paris Match magazine, she knew the protocols of the world of politics. "I know politics, I know the media," she boasted.
Within days Trierweiler, 47, became known as the "first girlfriend", and inherited her predecessor's support staff and bodyguards.
Trierweiler, the mother of three teenage boys, has also insisted she would keep working as a journalist at the weekly Paris Match. Her editors agreed, but restricted her to the arts and culture beat.
Last week's Paris Match carried Trierweiler's review of a new French-language biography of America's former first lady social reformer Eleanor Roosevelt. The review's opening line is : "Look at that! A journalist first lady isn't a novelty."
Royal, meanwhile, is milking the situation for all she can. She has described the violent nature of the tweet and says she felt "murdered".
"I demand respect as the mother of a family whose children hear what is said. I didn't want to respond in the heat of the moment because I am fighting a tough political battle and I need to stay in a good state of mind," she said.
The shrinks are poring over why Trierweiler would risk bringing her jealousy into the public domain.
"We are watching the actions of someone who, as we say, is going mad, and is showing their malaise," said psychoanalyst Serge Hefez.
But "Tweetgate" also sheds light on the undefined role of France's first lady. Trierweiler says she wants to maintain her independence and right to free expression, a posture that is hard to square with her position as partner and adviser to Hollande.
For now, Royal has much to lose. Hollande moved out of the family home in 2007 within days of Royal losing to Sarkozy. Since then the public has learned that Hollande - whose nickname is "Flanby", after a soft, bland caramel dessert - had in fact been in a relationship with Trierweiler since 2005.
Earlier this year Royal scored just under 7 per cent of votes in the Socialist Party's primaries for the presidential candidate. Opinion polls predict a victory for her dissident socialist opponent this weekend by at least 15 percentage points. That result would be humiliation for Royal.
It would also mean fewer invitations to state glittering occasions, leaving the red carpet for the first girlfriend and her partner.
