Japan needs to be part of a process that could shape the rules of global trade, Mr. Abe told the annual convention of his ruling Liberal Democratic Party in Tokyo on Sunday, while also pledging to defend sensitive sectors of the home economy.

"I will protect Japan's agriculture and its food at all costs," Mr. Abe told the 3,500 lawmakers, party members and guests gathered in Tokyo. "I ask you to please trust me, believe in me." An election for Parliament's upper house, which his LDP doesn't now control, is set for July.

Backed by strong public-approval ratings since returning to power late last year, the prime minister announced on Friday he would take Japan into negotiations to join the U.S.-led Trans-Pacific Partnership, part of his long-term growth strategy.

Since the LDP won a sweeping general-election victory in mid-December on a platform of aggressive monetary easing and fiscal spending, the Nikkei Stock Average has climbed 30% and the yen has weakened 13% against the dollar, helping exporters and improving business and consumer sentiment.

Japan hopes to join the TPP negotiations, which already involve the U.S., Canada, Australia, New Zealand and seven other Pacific nations, in July. But it will be over the loud objections of farmers who fear an influx of cheap imports¡ªand LDP lawmakers who depend on those farmers' votes.

Mr. Abe argued that the benefits from growing regional trade outweigh the risks.

"The world is steering toward an open economy, and Japan cannot be left behind," he said, adding the country should lead the TPP negotiations with the U.S., a process that will lay down the basic rules for other regional trade frameworks.

The LDP sought to ease opponents' concerns by pledging in the party policies adopted Sunday to work to protect "national interests, especially in the areas of agriculture, forestry and fisheries."

But in a sign of how difficult it may be to wall off selected sectors of the economy, LDP Secretary General Shigeru Ishiba suggested on TV Saturday that the negotiations could lead to lower tariffs on agricultural products.

While Japan will focus on protecting the likes of rice, barley, beef and pork, he said, that doesn't lead to the argument "that tariffs can't be lowered even by 1%."

The government forecasts that the value of the agricultural sector's output could decline by ¥3 trillion ($31.5 billion) if Japan joins the TPP¡ªbut that gross domestic product would likely gain about two-thirds of a percentage point, or ¥3.2 trillion.

The man in charge of Japan's negotiations, Economy Minister Akira Amari, said on TV Sunday that the estimates on the agricultural impact are based on the "extreme premise¡­where tariffs are immediately lifted and no domestic measures are taken."

"I expected figures to be far worse, but even with such premises, we see that the GDP will rise," Mr. Amari said.

Friday's decision to join the talks coincided with parliamentary approval of new leadership for the Bank of Japan. The prime minister has made new central bank Gov. Haruhiko Kuroda and the two new deputy governors responsible for pushing the monetary easing that Mr. Abe considers essential to lift the nation out of long-running deflation.

Mr. Kuroda, a former finance-ministry bureaucrat who has been serving as president of the Asian Development Bank, recently told parliament he was willing to do everything necessary to realize the central bank's recently adopted 2% inflation target. Among the possible measures he mentioned were purchases of longer-term government bonds and riskier assets, as well as an earlier start to the bank's open-ended asset-purchases, now scheduled for 2014.

Many market participants expect fresh easing steps to come out of the BOJ's next policy board meeting in April. Some even think they could be announced earlier, soon after Mr. Kuroda assumes his new job March 20.