Critics accuse him of seeking to destabilise Prime Minister Giuseppe Conte's administration in a bid for bigger cabinet roles – at the worst possible time.
But he claims he is following his conscience as Italy grapples with more than 82,000 dead from the coronavirus pandemic, a dire economic outlook, and an increasingly vocal opposition.
As yet it seems unclear how Renzi might benefit, with many of his former coalition partners vowing to never share power with him again, and his party logging just 2.4 percent of support in opinion polls.
Boy scout turned 'Wrecker'
Renzi rose though Italian politics as a youthful reformer, out to shake up Italy's political establishment.
However, he soon earned the nickname “Rottamatore” (“the Wrecker” or “the Scrapper”).
Serving as mayor of his native Florence from 2009 to 2014, he raised great hopes when he was elected leader of the centre-left Democratic Party (PD) in December 2013.
A former boy scout, without the ex-Communist baggage and grey demeanour of most of his predecessors, Renzi exuded self-confidence and dynamism.
In 2014, at the age of 39, he became Italy's youngest-ever prime minister since Benito Mussolini, and led the PD to a historic win in European Parliament elections, with almost 41 percent of the votes.
Matteo Renzi as newly-appointed prime minister in 2014. Photo: Alberto Pizzoli/AFP
But then the lucky streak of a man who, as a teen, won 48 million lira (24,800 euros) on the Wheel of Fortune TV quiz show began to run out.
Under Renzi, Italy liberalised labour laws, modernised the school system, legalised same-sex unions and cut taxes on low-income workers.
But his centrist policies and increasingly arrogant style antagonised trade unions and the left-wing of his party, as well as the broader public.
Twists and turns
In December 2016, he led a referendum campaign for constitutional reforms, which he hoped would bring some stability to Italy's notoriously volatile politics.
But it backfired, and when he was defeated by a 59-41 percent margin, he resigned.
Despite promising to quit politics over that failure, he stayed on as the PD's party leader – and in recent years has been involved in a remarkable series of political twists and turns.
After the 2018 elections, he opposed moves to form a coalition between the PD and the populist Five Star Movement (M5S), who went on to join up with the far-right League.
But after that coalition collapsed, he helped negotiate a M5S-PD government – before stunning observers by leaving it, and setting up his own party, Italia
The centrist group has flopped in opinion polls, prompting accusations that Renzi's attacks on the current government are motivated by a desire to get back some of the political power he has lost.
The most serious charge he levelled against Conte was that the premier lacked vision on how to spend the more than 200-billion-euros ($241 billion) in EU recovery funds.
The prime minister is trying to keep his government together without Renzi, going to the Senate on Tuesday for a vote of confidence – in which Italia
Viva will abstain.
Renzi continues to insist his motives are pure, declaring Monday: “When the fog of fake news clears… we will understand that the problem is not my character, but the failure to reopen schools, high Covid mortality, the most serious economic crisis in Europe, the vaccination delay, a Recovery Plan not worthy of our country…”
By AFP's Alexandria Sage and Alvise Armellini
Matteo Renzi listens to Italian Prime Minister Giuseppe Conte addressing senators on January 19th, 2021. Photo: AFP