By Gavin Jones
ROME (Reuters) - Fresh from its successes in last month's local ballots, the anti-establishment 5-Star Movement (M5S) is now Italy's most popular party and would easily win if a national election were held today, three opinion polls showed this week.
One survey by the Ipsos agency, published in daily Corriere della Sera on Wednesday, gave M5S 30.6 percent of the vote, up from 28.9 percent in April, while Prime Minister Matteo Renzi's Democratic Party (PD) fell to 29.8 percent from 31.1 percent.
The polls make especially grim reading for Renzi because he faces a do-or-die referendum in October on constitutional reform, having promised to resign if he loses the vote. M5S and all other opposition parties are campaigning against the reform.
His ebbing popularity coincides with growing friction inside his coalition, with a group of disaffected lawmakers from the small center-right NCD party threatening to join opposition ranks in a move that would make it extremely hard for the government to pass legislation in parliament.
Adding to the gloom, the NCD leader and interior minister, Angelino Alfano, is embroiled in a scandal over alleged influence peddling by family members. He has denied any wrongdoing, but opponents have demanded his resignation.
"If Alfano falls, we go to elections," the Huffington Post's Italian website said, in a headline across its front page.
Italy's next national election is not scheduled until 2018. All recent polls have shown that under a new, two-round electoral system pushed through by Renzi last year, 5-Star would easily win in a likely head-to-head with the PD.
The strength of M5S in second-round ballots was reflected in mayoral elections last month, when it won in 19 of the 20 run-offs it contested, including the capital Rome.
This is because M5S has no clear ideological identity. So, in the second round, left-wing voters support an M5S candidate rather than a right-wing one, and right-wing voters back an M5S candidate over a left-wing one.
The 5-Star Movement, founded by comedian Beppe Grillo in 2009, bases its appeal on the fight against Italy's rampant corruption and pledges to break down the privileges of its political and business elite.
Grillo has taken a back seat over the last year in favor of a new group of young leaders led by 30-year-old Luigi Di Maio, who is widely expected to be the party's candidate for prime minister at the next election.
Di Maio is now Italy's most popular political leader with an approval rating of 30.6 percent, compared with 27.2 for Renzi, according to a Euromedia survey.
(Writing by Gavin Jones and Crispian Balmer; Editing by Louise Ireland)