Four heavyweights of a far-right Turkish nationalist party were expelled on Friday for opposing the party's line in favour of expanding President Recep Tayyip Erdogan's powers.
On April 16 Turkish voters will cast ballots in a referendum on a bill that would create an executive presidency, which the government argues would be like that of France or the United States.
The government also argues the changes will ensure stability and create more efficient governance but opponents say it would lead to one-man rule and further inflame tensions in its diverse society.
The disciplinary committee of the Nationalist Movement Party (MHP) removed three lawmakers: Ismail Ok, Yusuf Halacoglu, Nuri Okutan as well as rising star and party member Sinan Ogan, MHP said in a statement.
They are accused of violating party rules after publicly voicing their opposition to MHP supporting constitutional changes proposed by the ruling Justice and Development Party (AKP) co-founded by Erdogan.
The four were expelled over misusing their rights of being members to deliberately harm the party as well as being accused of acting against party unity and leader, state-run news agency Anadolu reported.
The MHP is led by Devlet Bahceli, who first became leader in 1997, and is the country's fourth-largest faction in parliament.
Bahceli met regularly with Prime Minister Binali Yildirim in recent months and lent his support to the AKP bill, giving the party the necessary votes to call a referendum.
Those unhappy in the party accuse Bahceli of cosying up to Erdogan for his own ends, with many speculating -- without any proof -- over whether he has been promised a vice presidential role under the new system.
The role of premier would be axed while there would be one or more vice presidents.
Less than a year ago, Bahceli's position was fragile after former interior minister Meral Aksener was trying to run for the party's leadership.
The attempt failed and she was expelled from the party last September.
Polls show the April referendum will be a tight race with some pro-government surveys suggesting the changes will be approved while opposition newspapers publish polls showing the "No" side ahead.