ONE word can make all the difference. For the Italian left it was Chi? (Who?). On January 4th Matteo Renzi, the new leader of the centre-left Democratic Party (PD), was asked about a suggestion by the deputy finance minister from the PD, Stefano Fassina. “Chi?” was his contemptuous reply. Mr Fassina promptly resigned.

Thus began a difficult few days for the Italian left (Mr Renzi’s predecessor, Pierluigi Bersani, later went to hospital after suffering a cerebral haemorrhage). Mr Fassina mattered more than most ministers, as keeper of the PD’s left-wing conscience and an influence over the economic policy of Enrico Letta’s left-right coalition. His departure reveals the tensions in the PD caused by the election of the business-friendly Mr Renzi in December.

Like many of his generation, the 38-year-old mayor of Florence despairs of Italy’s political establishment. Ignoring the PD old guard as well as younger associates like Mr Fassina, he has formed an executive composed entirely of his own allies. Mr Fassina’s resignation was largely ideologically motivated. But, as he told an interviewer, it was also no fun being in a government under constant attack from Mr Renzi, leader of the main party ostensibly supporting it. Mr Fassina’s suggestion, which prompted Mr Renzi’s caustic Chi?, was for a reshuffle so that some of the mayor’s chums could find out how hard it is to run a country.

Italy has a long tradition of party leaders pulling the strings from outside the cabinet. What is less common is for a party leader to deliver almost daily lectures like those Mr Renzi has fired at the government since his election, painting it as representative of the old way of doing things. Mr Letta, though no grey beard at 47, was first a minister in 1998. His main ally, Angelino Alfano, was Silvio Berlusconi’s right-hand man before splitting last year from the media tycoon to form his own party, the New Centre Right (NCD).

Mr Renzi wants to impose on the NCD a formal pact, like that underlying the grand coalition in Germany, but only for 12-15 months. Its centrepiece would be a reform of employment law to give young workers permanent jobs, but with reduced protection at the outset. The young mayor’s strength lies in an implicit threat to bring down the government by withdrawing PD support. But that assumes he can command his lawmakers—and Mr Renzi sits at the rightmost edge of the party he leads.

Refusing to apologise for the insult that pushed Mr Fassina out of government, he said he was answerable not to party factions, but to the 3m Italians who voted in a primary election last year that was open to all. Mr Renzi’s following in the PD is probably more accurately reflected in the 39% who backed him in a 2012 leadership ballot that he lost to Mr Bersani. But even those who supported him in December could soon tire of his fault-finding.

That might be a reason for Mr Renzi to carry out his threat soon, forcing an election in which the fledgling NCD would struggle for visibility as the PD squared off against Mr Berlusconi’s Forza Italia party. The snag is that, after a Constitutional Court decision in December, Italy needs a new electoral law before the next vote. And Mr Renzi probably needs Mr Berlusconi’s help to draw one up. Though convicted of tax fraud and expelled from the Senate, the former prime minister still has cards to play.