Mr. Renzi promised to push through economic change in Italy. Agaton Strom for The Wall Street Journal
NEW YORK—Italy won't successfully push back against austerity policies in Europe until it shows it is credible in driving through economic overhauls at home, Prime Minister Matteo Renzi said Wednesday.
The decisive showing by Mr. Renzi's center-left party in May's European elections left him one of the Continent's strongest leaders, propelling him to the forefront of a growing campaign to relax austerity measures to foster economic growth.
In Italy's case, Mr. Renzi has argued for a looser interpretation of European Union budget strictures that would allow the government some elbow room to spend on much-needed investments that could stimulate growth.
Italy's huge public debt, which is nearly 135% of gross domestic product, and an EU requirement to keep its budget deficit under 3% of GDP mean Rome must keep a tight lid on public spending.
In an interview with The Wall Street Journal, the prime minister said he would never convince Berlin or Brussels to switch their approach unless he could deliver on his promises to improve Italy's own competitiveness.
"Let me be very clear. I think austerity without growth is a mistake," he said. "But for me, it's impossible to explain that position without a very strong project of reforms in my country. I'm not believable if I am not able to realize reforms in Italy, I know."

Mr. Renzi's seven-month-old government is under heavy pressure to pick up the pace of structural change, with the Italian economy—the eurozone's third-largest—now mired in its third recession since 2008.
While the 39-year-old premier became popular by promising to shake up the country's much-discredited political class along with the sclerotic economy, his legislative plans have met stiff opposition from vested interests such as Italy's large civil service and its labor unions.
As a result, seven years into the financial crisis, Italy has implemented very few of the "urgent" economic reforms the European Central Bank called for three years ago. Those included new labor laws making it easier to hire and fire workers, an overhaul of its slow-moving bureaucracy and the liberalization of closed professions such as notaries and taxi drivers. As a result, some economists predict that Italy's economy will stagnate for years to come.
Italy's economic weakness also poses a serious threat to the revival of the eurozone at large.
Mr. Renzi, a former mayor of Florence who took an outside route to political power in Rome, insisted he would press on with his agenda.
"My commitment is very clear. Realize the reforms independent of reactions. Labor market reform is [a] priority in Italy and if the unions are against, for me it's not a problem. I continue," he said. "Maybe I can lose the next election but for me it's important not to lose this opportunity."
Mr. Renzi reiterated that his government wouldn't raise taxes to fund his ambitious program—a fear widespread among Italian businesses which already pays some of the highest taxes in the developed world.
"I absolutely exclude the possibility of new taxation in Italy," he said.
Write to Laurence Norman at laurence.norman@wsj.com and Deborah Ball at deborah.ball@wsj.com
Corrections & Amplifications
Italy's public debt is nearly 135% of gross domestic product. An earlier version of this article incorrectly said it was nearly 138% of GDP. (Sept. 26, 2014)