WASHINGTON (Reuters) - A group of Democratic lawmakers on Thursday urged President Barack Obama to insist on keeping a 2.5 percent tariff on Japanese autos and a 25 percent tariff on Japanese trucks if the United States and Japan enter into free trade talks.

The plea came one day before Japanese Prime Minister Shinzo Abe is expected to announce Japan's interest in joining talks on the Trans-Pacific Partnership (TPP), a proposed free trade deal between the United States and currently ten other countries.

That prospect concerns Ford and other Detroit-based automakers, who fear losing more sales to Japanese imports.

"In an industry with razor-thin profit margins, the elimination of the 2.5 percent car tariff (as well as the 25 percent truck tariff) would be a major benefit to Japan without any gain for a vital American industry," the lawmakers said in a letter to Obama.

The group included Representative Sander Levin, the top Democrat on the House of Representatives Ways and Means Committee. He is from Michigan and one of the fiercest defenders of the U.S. auto industry in Congress.

"What the letter does is sound an alarm about Japan's participation" in the TPP, Levin said in interview.

He said he was skeptical that negotiations could tear down regulatory and other non-tariff barriers that he said have long kept American autos out of Japan's market.

(Reporting by Doug Palmer)