Devlet Bahceli's Nationalist Movement Party is the smallest of three opposition groups in parliament and the only one that has made a decision to back a "yes" vote in a referendum on Sunday to consolidate more powers in the presidency.
For Erdogan, 63, a presidential system has been a long-time dream. Erdogan's supporters reject such charges, saying the 18 constitutional amendments being put to a simple "Yes/No" vote contain sufficient checks and balances, such as the provision that a new presidential election would be triggered should the president dissolve parliament. Erdogan's fervent supporters see his drive for greater powers as the just reward for a leader who has put Islamist values back at the core of public life, championed the pious working classes and delivered airports, hospitals and schools.
He has crisscrossed the country to hold mass rallies and led an often abrasive and divisive campaign, accusing his opponents of siding with "terrorists".
The U.S. has always been allied with Turkey and has appeared to support Erdogan in the past - the Obama administration denounced last year's coup attempt against him. "We have paid dearly for these delays".
Erdogan and government officials are accused of using state resources and official functions such as openings of infrastructure projects to campaign in favor of the changes. "Now the Organization for Security and Co-operation in Europe says if the result is "yes", that means there are a lot of problems".More news: Islamic State's official Mufti killed in Mosul strike
TRT World's Hasan Abdullah reports on campaigning in Ankara. Over two weeks spent observing referendum campaigning, the OSCE found that supporters of the "No" campaign faced bans, police interventions and violent clashes at their events.
"MHP voters feel deceived, they feel they have been fooled", said Sinan Ogan, a leading member of a camp within the MHP that wants to see Bahceli ousted.
Former MHP lawmaker Nuri Okutan - dismissed at the same time as Ogan - said that if a "Yes" vote was agreed the party would end up losing all significance as an opposition force. "You can not meddle in what happens if "yes" wins or if "no" wins". The Islamic State (ISIL) is believed to be behind several of these incidents; the Kurdistan Worker's Party, known as the PKK, and groups associated with it have claimed responsibility for others. So they say they saw "yes" everywhere and did not see "no".
If approved in the referendum, the changes take effect with the next general elections slated for 2019.