Prime Minister Shinzo Abe's Liberal Democratic Party faces a major test of its support from farmers in the coming Upper House election, the first nationwide vote since Japan and 11 other countries signed a trade pact that opens the agricultural sector to increased international competition.

The Trans-Pacific Partnership free trade agreement is a key growth policy for Abe's government, enabling Japanese companies to make deeper inroads into overseas markets to make up for shrinking domestic demand amid the country's declining population.

But farmers have persistently opposed the trade initiative since Japan joined talks on the pact in 2013, saying it will put the country's heavily protected farm sector into fierce competition with products from major agricultural exporters such as the United States and Australia.