The ruling and opposition parties locked horns over the Trans-Pacific Partnership free trade agreement on Tuesday, kicking off Diet deliberations on ratifying the pact. It is likely to be the predominant focus of debate until June, when this ordinary Diet session ends.

In a question-and-answer session, the Democratic Party and the Japanese Communist Party denounced the pact as unworthy of deliberation.

They pummeled the government, apparently with an eye on the Upper House election this summer, arguing that the government has underestimated the likely economic damage from TPP to the nation’s agriculture and fisheries industries. They said it also has failed to disclose information about how the negotiations took place.

Meanwhile, the ruling Liberal Democratic Party and Komeito, as well as the minor Osaka-based opposition force Osaka Ishin no Kai, praised the TPP deal. It was signed on Feb. 4 after months of marathon negotiations among 12 countries.

The government said TPP will boost the nation’s gross domestic product by ¥13.6 trillion a year and set a model for free trade rules across the Asia-Pacific region, including the United States. It needs to be ratified by a number of signatories before it can come into force.

The government has estimated the cost to the nation’s agricultural and fisheries sectors at between ¥130 billion and ¥210 billion a year.

Opposition parties have refused to start deliberation at a special Lower House committee on the TPP agreement. Instead, they are demanding that Akira Amari, the former top TPP negotiator who resigned in a February over a political funding scandal, be summoned for Diet questioning.

Amari was the only minister involved in numerous TPP negotiations with partner countries, the details of which were not revealed.

He stepped down in an alleged money-for-favors scandal involving some of his secretaries. Since then he has refrained from attending any public events, including Diet sessions, citing health reasons.

“Deliberations over the TPP pact will be more important than ever those over any other free trade agreements. But the former minister who was in charge of the negotiation won’t attend the deliberations,” said Shiori Yamao, policy chief of the Democratic Party during the Lower House session.

He said if Amari is not present it will not be possible to debate the matter to the level that voters demand, he said.

The government has said it did not keep records of what was said at the closed-door ministerial-level negotiations that Amari attended.

Nevertheless, the ruling parties have refused to summon Amari as a witness, apparently fearing damage at the summer polls from an appearance in public by the scandal-tainted minister.

The ruling LDP-Komeito coalition occupies over two-thirds of the Lower House and holds a majority of the Upper House, which gives it the ability to ratify the pact and related bills regardless of opposition from other parties.

However, the overall fate of TPP remains uncertain as all major candidates in November’s U.S. presidential election campaigns, including former Secretary of State Hillary Clinton, have expressed opposition to it.

If the U.S. Congress refuses to ratify the deal the 12-nation pact will not go into effect.

Ratification is needed by at least six countries that account 85 percent or more of the bloc’s total gross domestic product. The U.S. alone accounts for about 60 percent of the total.