Ms Aubry, the Socialist Party leader and a former labour minister who was behind the cutting of the French working week to 35 hours, yesterday told supporters in Lille, where she is mayor, that she wanted to boost France's competitiveness while protecting low-income citizens.
She will compete with former party leader Franois Hollande, ranked ahead of her in the polls, in a selection contest that will wind up in October, six months before the election.
Both candidates represent an old-style left, in contrast with the centrist former IMF chief Dominique Strauss-Kahn, who was the party's top hope for 2012 until his shock arrest last month for allegedly trying to rape a New York hotel maid.
However Ms Aubry could stick more closely to the left's stated programme of creating subsidised jobs, revisiting a 2010 pension reform and scrapping tax breaks than Mr Hollande, who paints himself as a modern thinker and more centrist.
In a speech to announce her election bid, Ms Aubry took a swipe at conservative president Nicolas Sarkozy, who is resented by many voters for passing measures to ease taxes on the wealthy. "Behind what looks today like energy, but could be seen as restlessness, the reality is unfair policies that solely benefit the most privileged. It is time that changed," Ms Aubry said.
She is a no-frills politician, even describing herself as "a fuddy-duddy", who would offer a stark alternative to the "bling bling" of Mr Sarkozy, who loves to be at the centre of the world stage and whose wife Carla Bruni is a former supermodel.