A French Polynesian assembly member, Isabelle Sachet, has quit the Tahoeraa Huiraatira Party to join the group led by the president, Edouard Fritch.
Ms Sachet says she wants to back the president as he needs a majority to pass the budget.
Her move gives the government, which is made up of mainly Tapura Huiraatira politicians, the backing of 28 assembly members - one short of a majority.
Her departure is the latest amid a rift within the once dominant Tahoeraa which began last year when its leader, Gaston Flosse, was convicted for corruption and forced to leave office.
He was succeeded by Mr Fritch at the helm of the government.