U.S. lawmakers are drafting battle plans to punish foreign rivals accused of currency manipulation but a growing chorus of critics argue the battle is already won and warn the push may derail a massive trade pact. A bipartisan bill introduced this month and proposals from other lawmakers seek to stop trading partners such as China and Japan from manipulating exchange rates to make their goods cheaper for overseas buyers. The trigger for the new currency offensive is a nearly completed Trans-Pacific Partnership (TPP) trade pact. It encompasses 12 Pacific Rim trading partners including Japan, whose stimulus program has helped lop off a third of the yen's value since late 2011, worrying U.S. automakers like Ford Motor Co. And China is seen by many Americans as a great stealer of jobs, following years in which Beijing weakened its currency to undercut American manufacturers. Insistence on a currency chapter in the deal is widely seen as a "poison pill" which could derail the entire pact and reflects many lawmakers' distrust of more open trade. "You always need a bogeyman, someone to blame your troubles on," said Jeffrey Frankel, a Harvard professor who was a top economist in the Clinton White House. A series of respected economists and former top officials have voiced concern that currency restrictions will derail the trade pact, although some are also lined up on the other side. "Almost certainly, the effort would be a classic deal breaker," former Treasury official Ted Truman wrote in a blog post last week. Federal Reserve Chair Janet Yellen spoke out against the currency proposals, telling senators such rules could "hobble" monetary policy. Evidence these days is scant that either Japan or China, America's top trading partners in Asia, are engaged in currency manipulation following decades of U.S. pressure to lay off. Even backers of currency rules concede Japan is not manipulating its currency now, although they warn it could again in future. Japan's yen is weakening thanks in part to the same policies pursued by the U.S. Federal Reserve and it maintains that it has not intervened in currency markets since 2011. Tokyo signed on for a 2013 Group of Seven agreement, pushed by Washington, in which it agreed to use monetary policy only to boost domestic growth. The U.S. Treasury has been increasing pressure on South Korea, another potential TPP partner, to let the won appreciate, but does not consider Seoul a currency manipulator. China, according to the U.S. Treasury, has stuck by a deal struck in 2014 that it would not intervene and the yuan has appreciated 57 percent in real trade-weighted terms since 2004, more than almost any other currency tracked by the Bank for International Settlements. While Beijing could choose to reverse course and the central bank still controls the exchange rate by setting an official trading band, economists say slowing economic growth, interest rate cuts and capital outflows mean market forces, rather than purchases of dollars, are weakening the yuan. China now appears to be intervening to prop the currency up, as evidenced by the slight decline in its near $4 trillion in dollar reserves during the fourth quarter. "We have to get used to the new reality," said David Dollar, who was the U.S. Treasury's top emissary to China between 2009 and 2013, a time when America was actively pressing Beijing to stop weakening the yuan. (Reuters)