In the frantic final round of campaigning which ended on Friday, Renzi's domestic rivals vowed to knock down referendum proposals to streamline parliament and force the centre-left leader out of office.
The prime minister is hoping for a last-minute turnaround in voter sentiment in favour of a "Yes" vote when Italy goes to the polls on Sunday.
All media comment on the vote is now banned until polling ends at 11:00 pm(2200 GMT) on Sunday night.
With #silenzioelettorale (electoral silence) widely in evidence on Twitter, most Italians online were rejoicing at the end of a highly-charged campaign.
Politically and economically, the stakes are high.
After Britain's vote to leave the EU and Donald Trump's presidential triumph in the United States, Renzi is being portrayed as next in line to suffer a populist backlash from fed-up and forgotten voters.
His pledge to quit if he loses the vote has focused the campaign on his record, exacerbating fears of political instability and economic turbulence in the eurozone's third-biggest economy should he be forced out.
At stake on Sunday is whether to slash the size and powers of the second chamber Senate and transfer other powers from the regions to the national government.
Renzi says this will mean more effective leadership of a country that hashad 60 different governments since the constitution was approved in 1948.
But it seems certain some disgruntled voters will vote "No" as a protest --either against Renzi or the years of economic stagnation.
The proposals have come under fire from opponents who see them as ill-considered and potentially opening the door to the kind of authoritarian rule the constitution is designed to prevent.
Some 50 million Italians are eligible to vote, with opinion polls showing many people still undecided.
The last permitted polls, published on November 18, gave the "No" camp at least a five-to-eight-point lead, with more than a quarter of voters undecided.
'Could undermine democracy'
In the Mercato Trieste, a food hall in a leafy middle-class neighbourhood of northern Rome, most of traders and shoppers on Saturday morning appeared to be either opposed to the reform or yet to make up their minds.
Egg seller Emanuela Carosi, wrapping free range eggs in bundles of newspaper, said she would definitely vote "No".
"The Constitution was written by chosen people, luminaries, not politicians like we have today. They are not up to it. And I'm worried that it could undermine our democracy."
Slicing Wagyu beef steaks at an upmarket meat counter, butcher Antonio Canestri said he also opposes the reform but may vote for it for fear of the consequences.
"I am afraid about what happens in the event of a 'No'. We know what we have now, we don't know what we will find with the reform, but I am worried about the possibility there could be economic chaos if the 'No' wins."
A large majority of the political class, from the far-left to the extreme right, have warned about the risk of concentrating too much power in the government's hands.
48 crucial hours
Former premier Silvio Berlusconi initially gave his blessing to the proposed reform but switched sides as the rising tide of opposition put Renzi's job on the line.
"Go out and vote so that Renzi doesn't become our boss and the boss ofItaly," he said late on Friday.
At the same time Renzi, was campaigning in Florence, where he was formerly mayor, urging supporters to convince the undecided.
"There are so many of them and we must go and seek them out, one-by-one, because everything will be decided in these 48 hours," he said.
The possibility that it could turn into a tight race has energised Italiansabroad, for whom voting ended on Thursday.
Expat Italians number around four million -- or 7.7 percent of the electorate -- and a strong showing there could likely boost Renzi's "Yes" camp.