In the letter, the senators argue that Japan has too many barriers in place to U.S. automakers to make them worthy of an invitation.
"The United States has attempted to address this imbalance in past trade negotiations, but it has been unsuccessful," they wrote.
"Japan has previously made concessions, including the elimination of tariffs on automobile imports. However, new barriers to trade have arisen to replace the old," the senators wrote. "We are particularly worried about the impact that Japan's inclusion will have on American carmakers and their workers."
They say that Japan has made concessions, including the elimination of auto tariffs, but then has followed up by putting other hurdles in their place.
"We believe it would be a mistake to invite Japan to join TPP at this time."
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The 13th round of negotiations between the 11 nations — the United States, Australia, Brunei, Canada, Chile, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam — begins Sept. 6 in Leesburg, Va.