By Crispian Balmer

ROME (Reuters) - If Italian Prime Minister Matteo Renzi loses his referendum on constitutional reform, the chances of the anti-establishment 5-Star Movement winning power will recede rather than grow.

Renzi has said he will resign if the Dec. 4 ballot goes against him and markets are worried that this might open the door to the opposition 5-Star, which has denounced the euro.

Latest opinion polls show the 'No' camp well ahead, pushing the yield gap between bellwether Italian and German 10-year bonds to two-year highs, partly out of concern over the seemingly unstoppable rise of the 5-Star.

However, analysts say Italy is more likely to return to old-style consensus politics after a 'No' vote, with traditional mainstream parties on the left and right sharing a common interest in keeping the maverick 5-Star in opposition.

The key to doing that will be a reform of the electoral law, which heavily favors the 5-Star and which Renzi said this week would have to be changed before the next election, which is due in 2018, regardless of what happens in the referendum.

"It is clear that as things stand, the present law would have brought the 5-Star to power. By changing it, many different electoral outcomes will open up," said Piero Ignazi, a political science professor at Bologna University.

The last three elections in Italy were staged using an electoral law that was so universally disliked it was known as the "pigsty" and was eventually ruled unconstitutional.

After many attempts at reform, Renzi finally succeeded last year, introducing the so-called Italicum which gave Italy a two-round voting system, with the leading two parties from the initial ballot heading into a decisive run-off, promising a sizeable parliamentary majority to the victor.

The law was originally thought to favor Renzi's Democratic Party (PD), but the 5-Star's standing has since risen and all recent polls say it would easily triumph in a two-round race.

COLD WAR POLITICS

Alarmed by these surveys, the PD said this month that it was willing to get rid of the run-off ballot and review how seats should be distributed in a simple one-round system.

The center-right Forza Italia (Go Italy) party of former prime minister Silvio Berlusconi supports this stance and is calling for a return to proportional representation.

"If no-one wins a clear majority, then the two (political) blocs will have to reach a (coalition) deal, as happens in Germany," Berlusconi told RTL radio on Wednesday.

The 5-Star has in the past ruled out any coalition deals to avoid compromising its ideals, meaning a pure proportional representation system could leave it perennially in opposition, like Italy's powerful Communist Party during the Cold War.

Despite being favored by the Italicum, the 5-Star is also clamoring for change, saying the current law is undemocratic.

"This shows people that we are different and don't do things purely out of self-interest, like the other parties," said 5-Star lawmaker Danilo Toninelli.

He has drawn up a draft electoral law that also foresees a return to proportional representation, but with a very high vote threshold for entering parliament which will sweep away smaller parties and reward the bigger ones.

This system might initially result in broad left-right coalitions, but Toninelli said voters would eventually realize that such pacts do not work and turn in droves to the 5-Star.

"We are thinking in the long term ... but they will never accept our proposal. They will draw up an electoral law that is the most anti-5-Star they can come up with," he told Reuters.

(Editing by Robin Pomeroy)