Dhivehi Rayyithunge Party (DRP) leader Ahmed Thasmeen Ali resigned from the post on Monday.
The Kendhoo MP resigned as some top members are rumoured to be defecting to opposition Maldivian Democratic Party (MDP).
Thasmeen had been the running mate of president Mohamed Waheed Hassan in the first round of presidential elections held September 7 which had been later annulled by the Supreme Court.
After Waheed withdrew his candidacy from the re-vote, Thasmeeen and his party had joined forces with MDP to back opposition leader Mohamed Nasheed.
However, Nasheed had lost the presidential elections to Progressive Party of Maldives (PPM) candidate Abdulla Yamin in a runoff on Saturday.
Thasmeen had also been the running mate of former president Maumoon Abdul Gayoom in the 2008 presidential elections. Nasheed however, won the elections in a runoff against Gayoom.
Following a rift with Thasmeen, Gayoom had quit DRP and later formed PPM.