It's now predicted to have lost so much support it could be wiped out of parliament.
In Turkey, people will vote in a referendum on a new political system on Sunday.
In 2006, Erdogan began to talk about the need for Turkey to move from a parliamentary system to an "American-like" presidential one.
It also alleged that Ankara's senior officials, including Erdogan himself and Prime Minister Binali Yildirim, equated "no" campaigners with the mid-July 2016 failed coup plotters or terrorist organizations, noting that the referendum would be conducted under an extended state of emergency imposed following the attempted putsch.
Burak Bilgehan Ozpek, associate professor of worldwide relations at Ankara's TOBB University, said a "Yes" vote would mean the end of the opposition within the MHP. That "unduly invests the weight of the decisions and the power of the executive on the president".
"It's going to be a very close call, and both "yes" and "no" are equally probable as outcomes", Han said.
The OSCE Office for Democratic Institutions and Human Rights conducted the limited referendum campaigning observation mission in Turkey at the invitation of the country's authorities.
Turkey has arrested or suspended more than 140,000 people since the coup attempt and has jailed the most journalists out of any country in the world. In 2010, Erdogan's government took several hundred officers to court on charges of planning to overthrow the government.More news: Thunder star Russell Westbrook resting against Timberwolves
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Gulen denies any ties to the coup attempt.
Turkish President Tayyip Erdogan and Russian President Vladimir Putin agreed on Thursday to support an investigation by the Organisation for the Prohibition of Chemical Weapons into the chemical weapons attack in Syria last week, Turkish presidential sources said.
For Erdogan, 63, a presidential system has been a long-time dream. The changes will allow the president to dissolve the parliament which will trigger the renewal of presidential and parliamentary elections simultaneously.
He expressed confidence that the new presidential system would be approved, saying there were no longer undecided voters. The referendum is seen by many as the ultimate test of President Recep Tayyip Erdogan's popularity.
The vote has become somewhat of a referendum on Erdoğan himself, who a year ago survived a failed coup attempt and emerged stronger than ever. "We have paid dearly for these delays". The opposition CHP Party is against the 18 proposed reforms.
Erdogan's comments referred to a recent interim report published by the OSCE, which will monitor the April 16 referendum, on the campaign atmosphere in the Anatolian country between March 17 and April 7.
"They said that if there is a "yes" vote, it would mean there are problems here". Who are you? First of all, you should know your place. "You can not meddle in what happens if "yes" wins or if "no" wins". Erdogan has accused European leaders of acting like Nazis for banning rallies on security grounds, while his opponents overseas say they have been spied on. 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.