ROME (Reuters) - Opinion polls are making increasingly grim reading for Italian Prime Minister Matteo Renzi less than three weeks ahead of a referendum on constitutional reform on which he has staked his political future.

Italian Prime Minister Matteo Renzi speaks during a rally in downtown Rome, Italy October 29, 2016. REUTERS/Remo Casilli

Of 32 polls published by 11 different pollsters since Oct. 21, every one has the ‘No’ camp ahead, and generally by a widening margin.

In three polls published on Monday the lead for ‘No’ ranged from five points, according to IPR Marketing, to seven points, according to Tecne, with EMG Acqua in the middle at 6 points.

These results exclude undecided voters, which are estimated at 25.9 percent by EMG Acqua and 16.5 percent by Tecne. The most worrying aspect for Renzi is that as the number of undecided voters declines, the lead for ‘No’ appears to be rising.

Bookmakers also hold out little hope for the 41-year-old premier, with Ladbrokes estimating a roughly 75 percent probability of a win for ‘No.’

The surveys are so one-way that attention is turning to what Renzi will do if he loses the vote on his plan to drastically reduce the role of the upper house Senate and take powers back from regional governments.

At the start of the campaign he repeatedly said he would resign in the case of defeat.

He then declined to confirm that, saying discussion of his own future deflected attention from the merits of the reform, but in the last few days he has once more began hinting that he will not try to hang on in power if he loses.

However, most pollsters continue to say the outcome of the Dec. 4. ballot remains uncertain.

They point out that opinion polls already proved notoriously wrong in the June referendum in which Britons chose to leave the European Union and most recently when Americans elected republican Donald Trump to the presidency on Nov. 8.

REASONS FOR CAUTION

Moreover, in Italy’s case there are some specific reasons for caution.

One is that the polls show that the ‘No’ vote is strongest in the south of the country, where turnout is normally lowest.

“It may be that a significant number of those in the south who say they are going to vote ‘No’ will end up staying at home,” said Federico Benini, head of the Winpoll agency.

Some 4.2 million Italian ex-pats who are eligible to vote but are not included in opinion polls could also come to Renzi’s rescue.

Benini forecast that of those that cast a ballot, up to 80 percent will back ‘Yes’ because they follow Italian politics less closely, tend to be less anti-Renzi and see it as broadly positive that the country is trying to reform.

However, he also said he expected only about 30 percent of Italians abroad to vote, meaning they will only be vital if ‘Yes’ is less than one point behind among domestic voters.

Another factor that could save Renzi is the wording of the question on the ballot sheet, which has been the subject of bitter dispute and legal cases because it mentions the more popular aspects of the reform but not the less positive ones.

Pollster Nicola Piepoli said as many as 8 percent of voter with little interest in politics could head to the polling stations without having previously made up their minds, and these may be swayed by what is written on the ballot sheet.

Pollster Renato Mannheimer said it was too soon to say whether Trump’s surprise election will affect Italian voters.

However, six out of seven polls published since the U.S. election have shown the ‘No’ lead widening compared with the previous survey by the same pollster.

Moreover, in a Winpoll survey published on Saturday, 52 percent of Italians said they believed Trump’s victory would favor ‘No’, compared with 42 percent who thought it would have no impact, and just 6 percent who said it would help ‘Yes’.

The final polls will be issued on Friday, as Italian law prohibits their publication in the 15 days before an election or referendum.