"Never have there been so many people undecided. The referendum match will be decided in the last 48 hours," the centre-left leader said in a Q and A session session on Facebook today.
Renzi, 41, is battling to defy opinion polls which point to his proposals to streamline parliament being rejected.
Such an outcome is expected to trigger the reformist premier's resignation after just under three years in office and plunge the country and Europe into a phase of political uncertainty.
The narrative has played strongly internationally but less so in Italy, where the merits of the proposed reform itself have been vigorously debated in a contest which has also focused on Renzi's record and personality.
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At stake 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 has vowed to quit if voters reject changes he says will mean more effective leadership of a country that has had 60 different governments since the constitution was approved in 1948.
But the proposals have also 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.
"This reform reduces the autonomy of local authorities and it concentrates too much power in the hands of the government without the necessary checks and balances," former prime minister Massimo D'Alema, a party colleague of Renzi's, told AFP.
Politically and economically, the stakes are high.
"If you want to abolish the privileges of the most expensive political caste in the world, you have to vote yes," the youthful premier said today.
Opponents insist the savings will be much smaller and that the legislative gridlock problem is exaggerated: Renzi did not have any trouble getting his controversial Jobs Act through parliament last year.
A No vote would bolster Italy's populist and far-right opposition parties, already buoyed by Trump's success.