Prime Minister Shinzo Abe's decision to join negotiations for an emerging regional trade agreement gained kudos from the business community and many city dwellers. But it remains a hot-button issue, and the prime minister faces opposition from both the left and the right of the political spectrum.

That tension, and the complex nature of the opposition to the Trans-Pacific Partnership agreement, were on full display Monday after Mr. Abe wrote a bluntly-worded post on his widely-read Facebook page about TPP protestors.

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