* Socialists endorse Mesterhazy to lead alliance
* He promises economic, social, democratic reforms
* Pledges to revise big nuclear deal with Russia
* Ruling Fidesz enjoys comfortable lead in polls
By Marton Dunai
BUDAPEST, Jan 25 (Reuters) - Hungary's leftist opposition alliance launched its election campaign on Saturday but faces an uphill struggle to defeat Prime Minister Viktor Orban's ruling Fidesz party on April 6.
The Socialists - the main force in the alliance - endorsed their leader Attila Mesterhazy to head the opposition grouping in the battle against centre-right Fidesz.
"We have an offer (to voters) that we talked through with our allies as well," Mesterhazy told a 10,000-strong crowd at his party's congress in Budapest.
"It is very simple. We need a responsible economic policy, a fair social policy and a strong democracy."
The Socialists' allies include former Socialist Prime Minister Ferenc Gyurcsany's Democratic Coalition, the Egyutt (Together) 2014 formation of another ex-premier, Gordon Bajnai, and former lawmaker Gabor Fodor's new liberal party.
However recent opinion polls suggest Fidesz enjoys a comfortable lead over the individual alliance parties combined.
Hungary is slowly returning to growth following two bouts of deep recession after 2008 brought on by the economic crisis, and the government has struggled to get employment and investment back on a stable footing.
Mesterhazy derided Orban for what he said was an "idiotic economic policy", a social policy that he said had left millions near or below the poverty line and forced hundreds of thousands of people to leave the country seeking a better life.
The government has said it saved Hungary from a Greek-style collapse after inheriting a fragile economy from the Socialists in 2010. It says Orban fixed the budget after years of chronic mismanagement, leading the country out of the European Union's fiscal monitoring programme by 2013.
Orban took over $14 billion in private pension fund assets to cut state debt, while taxing financial and energy companies heavily to finance tax cuts and fund the budget.
Fidesz has used several rounds of household utility price cuts to woo voters at the expense of mostly foreign energy companies, a programme the party said it would continue this year and make utilities a non-profit sector.
Mesterhazy dedicated much of his speech to a recent deal Orban signed with Russian President Vladimir Putin to expand Hungary's Paks nuclear power plant using Russian technology and a $13 billion Russian loan.
The Socialist leader said Orban did not have the authority to sign the agreement without prior consultation with experts and the public, and said if elected he would reopen the deal and make it dependent on proper consultations and a referendum.
He also mocked Orban supporters' recent mantra pledging "Hungary will not be a dominion" of Western interests.
"Viktor Orban executed a coup against the Hungarian people," Mesterhazy said. "He campaigned just last year saying we won't be a dominion ... except, for good money, a dominion of Russia."
Political Capital analyst Peter Kreko said: "The biggest problem for the opposition right now is there is no strong anger against the government that could mobilise opinion."
"The government did pretty well to calm the public and strengthen its image, especially with gas and electricity price cuts," he said.
"It seems that in less than three months the opposition should gain a million more voters than they have, in a country of 8 million voters. That is not impossible, but almost." ($1 = 0.7307 euros) (Reporting by Marton Dunai)