"As the Japanese government considers the question of whether, and under what terms, to seek entry into the negotiations, we too must consider our own workers and businesses."
They argue that prior efforts to open Japan's auto market have largely failed, even despite some changes, and have not allowed for more U.S. exports.
By comparison, other OECD countries with major auto sectors typically have an import share of more than 40 percent, including the U.S.
They also pointed to the fact that Japan was the second-largest source of the U.S. trade imbalance after China last year.
In fact, over the past five years, for every automobile that the U.S. exports to Japan. Japan exports more than 120 automobiles here.
The letter comes as Japanese Prime Minister Shinzo Abe is expected to formally announce on Friday the nation's interest in joining the talks after several years of hedging on the decision because of opposition within the nation, especially within their agricultural sector.
But expressing interest does not mean the nation would earn a spot in the talks, which have just concluded the 16th round of negotiations in Singapore.
During the recent discussions, the 11 member nations agreed that no nation would be permitted to wall off any sector of their economy to the trade agreement.
Still, according to news reports, eight TPP nations — Brunei, Canada, Chile, Malaysia, Mexico, Peru, Singapore and Vietnam — have already given their approval to Japan's entry.
Australia, New Zealand and the U.S. have not announced their decisions.
After Abe met with President Obama last month at the White House, the two nations appeared to set up a possible track to let Japan enter the discussions. But those talks did include possible exclusions and were clear that Japan would not have to agree to drop certain tariffs before entering the discussions.
At least one U.S. business group — the Business Roundtable — has suggested that Japan's entry would derail the aim to complete the deal this year and should be put off until the current nations wrap up an agreement. They argue that Japan should be let in after that, when it is prepared to meet the standards set out in a final deal.
The Australian Chamber of Commerce has recently raised objections to Japan joining TPP because its of struggles completing a free-trade agreement with Japan over agricultural issues.
The lawmakers, in their letter, also noted that Japan has other barriers such as currency manipulation, a discriminatory system of taxes and costly vehicle certification procedures for imported automobiles, among others.
"While some have compared this challenge to the one we faced with Korea, the Japanese auto market is more impenetrable, the history of formidable barriers and imbalanced trade is longer, and the magnitude of the problem is far greater than with Korea," they wrote.
"American automobile companies and workers — those at the forefront of today’s economic recovery — have been forced to compete for decades on a fundamentally unlevel playing field. Japan’s participation in TPP threatens to make matters worse."