On Sunday, citizens go to the polls to vote on a package of constitutional changes with vast ramifications for the country’s future. This comes amid huge social and political tension at home and upheaval in Turkey’s international allegiances, not least its increasingly troubled relationship with Europe.

But with campaigners on both sides anxious about the result remaining very much on a knife-edge, policy makers in Ankara and Brussels say that predicting the future of the relationship between the two capitals is nigh-on impossible.

One European diplomat compared it to “crystal gazing.” Whereas a Turkish official refused to make any predictions, saying that everything lay in the hands of “the highest level decision-makers.” In reality, this means the decision will fall to the mercurial Mr. Erdogan.

The referendum is the culmination of the Turkish president’s long-held desire to overhaul the role that he took up in 2014, after 11 years as prime minister.

The package of 18 amendments to the constitution would formally transform Turkey from a parliamentary system into a presidential one. It would abolish the role of prime minister and hand Erdogan sweeping new powers to appoint the cabinet, judges and to rule by decree.

Advocates argue that the new system will usher in a new period of stability and prosperity at a time when Turkey has been wracked by terror attacks and a violent attempted coup. All the while, the country has been struggling to reboot the economy after a slowdown in growth and a volatile currency.

Opponents warn that it will hand over unprecedented, unchecked power to a man who has increasingly pursued an intolerant, authoritarian style of leadership.

The indefinite president

According to some analyses, Erdogan could rule until 2034 under the proposed changes.

It is not the first time that Erdogan has tried to usher in these alterations. In parliamentary elections in June 2015, the focus on a presidential system was blamed for his party’s failure to win an outright majority for the first time since it swept to power in 2002.

After the outpouring of nationalism that followed his pugnacious response to the failed coup in July 2016, Erdogan felt confident enough to have another go.

Most observers assumed that the Yes campaign would glide easily to victory, not least because of the extraordinary campaigning conditions. Opposition parties have not voiced fears of large-scale fraud on election day, but there have been complaints of a deeply unfair campaign.

A state of emergency imposed after the coup attempt enabled local governors to limit gatherings and rallies on the grounds of safety.

The co-leaders of the country’s second biggest opposition party are in jail, accused of supporting terrorism, as well as over 100 journalists, with media coverage overwhelming skewed to favour the government's side.

Even with this tilted playing field, polling has suggested a remarkably tight race. Surveys released this week put the Yes vote at 51 to 54 per cent.

Some in the AKP, the conservative Justice and Development party, are worried that “shy” No voters may be hiding their true intentions.

“It’s still critical,” said one party insider, speaking earlier this week. “The atmosphere feels like the June elections — which were our worst ever.”

It is against this backdrop that Turkey plunged headfirst into a series of blazing rows with European countries that still technically view it as a candidate EU country.

Tulip Crisis

The centrepiece was a dispute with Holland, dubbed the Tulip Crisis, that exploded after Dutch ministers refused their Turkish counterparts permission to campaign among expat Turks in the days before the Netherlands held its general elections.

Erdogan described the Dutch government as “Nazi remnants and fascists.” He later accused German chancellor Angela Merkel of employing “Nazi tactics” and warned: “If Europe continues this way, no European in any part of the world can walk safely on the streets.”

Such rhetoric was partly aimed at galvanising undecided voters by playing on the deep-seated mistrust of European countries and their intentions towards Turkey - a tactic that seems to have had some success.

But Erdogan also feels genuine grievance at what he sees as the hypocrisy and false promises of the EU.

Although Turkey has faced no concrete punishment for the decline in human rights and freedom of speech, he still bristles at sporadic verbal rebukes.

The key question is whether he is really serious when he talks about formally ending Turkey’s already rapidly fading quest for EU membership.

Arch pragmatist

The Turkish president is an arch pragmatist who has shown himself willing and able to perform swift u-turns, such as last year’s rapprochements with Israel and Russia.

Some analysts believe that his talk of bringing back the death penalty - a move that would instantly guillotine the accession bid - is electoral bluster that will be swept under the carpet after Sunday’s vote.

Others say that Erdogan’s desire to recast the relationship with Europe is real, and that he would genuinely prefer to axe the accession bid and stick to the customs union deal that is already in place.

“What he clearly wants is a transactional relationship with no human rights criteria attached,” said Asli Aydintasbas, a senior policy fellow at the European Council on Foreign Relations. “Switching to that might be more truthful to the existing dynamic.”

Much depends on the outcome of Sunday’s vote, and whether Erdogan secures a resounding Yes, a narrow victory, or is left reeling from a painful set-back.

Aydintasbas predicted that, if the Yes side wins, Europe will probably be happy to muddle along with the status quo. But she warned: “What we don’t know is whether or not that is acceptable to Erdogan.”