RIMINI, Italy (Reuters) - Italy s anti-establishment 5-Star Movement, riding high in opinion polls, has named 31-year-old Luigi Di Maio to lead it into parliamentary elections next year that could see it win national power. Di Maio, deputy speaker of the lower house, is the party s most prominent and popular politician and had been groomed for the leadership in recent years by 5-Star s founder, the 69-year-old comedian Beppe Grillo. Boyish-looking and usually immaculately turned out in suit and tie, Di Maio presents a moderate image in striking contrast to Grillo, who is famous for his raucous tirades against Italy s ruling elite.  You have given me an enormous responsibility ... I assure you I ll carry it out with discipline and honor,  Di Maio told supporters on Saturday at the party s annual gathering in the Adriatic coastal town of Rimini.  I promise I will form a government team to be proud of for the first time in our history.     Grillo, who has so far acted as 5-Star s de facto chief, is now expected to gradually withdraw from the limelight.  I will always be like a dad for all of you, but I am old,  he told the gathering on Friday.     Di Maio, who has taken tough stances on law-and-order and immigration and is widely seen as on the right of the party which says traditional left-right labels have no meaning. He won by a huge margin over seven rivals in an online member ballot, held on Thursday and Friday, that reflected the movement s credo of internet-based direct democracy. His election was considered a formality because the other candidates were all little-known figures, mostly local councillors.  However, participation was much lower than the party had hoped, with just over 37,400 people casting a ballot, compared with more than 140,000 eligible.  Di Maio won 83 percent of the vote. The only people seen as having any chance against him decided not to run, opening the party up to accusations of failing to run a proper contest. Di Maio will now try to put that criticism behind him in the run-up to elections due by May next year. Most opinion polls, give 5-Star around 28 percent of the vote, just ahead of the ruling Democratic Party. However, that would not be enough to give it a majority in parliament unless it allies itself with another party, something it has so far always refused to do. The right-wing Northern League and Silvio Berlusconi s conservative Forza Italia (Go Italy) each have around 14 to 15 percent. 5-Star, founded by Grillo nine years ago as a protest movement, bases its appeal mainly on a campaign against corruption and vested interests. Its policies include the introduction of universal income support for the poor, boosting green energy and increasing the budget deficit to fund investments in infrastructure. Di Maio will need to broaden the movement s appeal and soothe barely disguised resentment among some 5-Star lawmakers over his dominant role in the party. His first challenge will be a vote in Sicily in November, which offers 5-Star the chance to win control of its first ever region. Victory could be a springboard to national success while defeat would be a major setback after the party spent months campaigning intensely on the island. (This version of the story has been refiled to add dropped words in paragraph 8) 