Beppe Grillo, leader of the populist opposition Five Star Movement, said: "Our country is dying and those responsible for governing go to the other side of the world to film a pro-Yes spot for the referendum.
"Every day that passes without the Five Star Movement in power is one more day of agony for Italians."
Italy's referendum is about power and who gets to wield it. Mr Renzi wants more of it for the elected government, as a way of reforming a political system he considers to be outdated and overcomplicated.
Mr Renzi proposes two specific changes: to simplify law-making by stripping Italy's upper house, the Senate, of most of its powers; and to give the elected government in Rome decision-making powers currently held by regions. He's threatened to resign if he loses (a threat that he's slowly begun to retract).
His opponents say the referendum is a simple power-grab. Prominent Italian politicians, including the two most recent prime ministers, Mario Monti and Silvio Berlusconi, have announced their decision to vote No.
Now Mr Obama has weighed in with his endorsement.
"How do you interpret the [Obama] endorsement?" wrote Paolo Pombeni, a journalist for Italy's Il Sole 24 Ore newspaper.
"It is the symbol of an American judgement of Italy's fragility. The US government thinks that political stability - and hence also economic and social cohesion - is at risk."
Current opinion polls show a race that is too close to call. Every vote will count. Well, apart from any cast from 1600 Pennsylvania Avenue.