TOKYO — Japan will seek to join negotiations for a free-trade pact with the United States and other Pacific nations, Prime Minister Shinzo Abe said on Friday, giving heft to talks that could encompass two-fifths of the world economy but also adding a country with demands and reservations that some participants fear will delay an agreement.

In an impassioned televised address, Mr. Abe portrayed the pact, called the Trans-Pacific Partnership, as Japan’s last chance to remain an economic power in Asia and shape the region’s future.

“Japan must remain at the center of the Asian-Pacific century,” Mr. Abe said. “If Japan alone continues to look inward, we will have no hope for growth. This is our last chance. If we don’t seize it, Japan will be left out.”

With strong opposition from Japan’s farming lobby and other powerful groups, Mr. Abe is taking a big political risk in embracing the free-trade talks. Japan’s largest agricultural cooperative has campaigned against trade liberalization. It says such a change would devastate the nation’s farms, a plea that has resonated in the wider public. A majority of the lawmakers in his own Liberal Democratic Party depend on the rural vote and object to the free-trade deal.