; and (2) a violation of any provision of the trade agreement described in subsection (a) relating to the matters described in paragraph (1) is immediately enforceable in accordance with the provisions of section 4. 4. Enforcement provisions (a) Withdrawal of tariff concessions If the President does agree to a modification described in section 3(a), and the United States Trade Representative determines pursuant to subsection (c) that— (1) a tariff or nontariff barrier or policy or practice of the government of a foreign country described in section 3(a) has not been reduced or eliminated; or (2) a tariff or nontariff barrier or policy or practice of such government has been imposed or discovered, the United States Trade Representative shall withdraw the modification until such time as the President transmits to Congress a certification described in section 3(b)(1). (b) Investigation (1) In general The United States Trade Representative, in coordination with the Department of Labor, shall initiate an investigation if an interested party files a petition with the United States Trade Representative which alleges the elements necessary for the withdrawal of the modification of an existing duty under subsection (a), and which is accompanied by information reasonably available to the petitioner supporting such allegations. (2) Interested party defined For purposes of paragraph (1), the term interested party means— (A) a manufacturer, producer, or wholesaler in the United States of a domestic product that has the same physical characteristics and uses as the product for which a modification of an existing duty is sought; (B) a certified union or recognized union or group of workers engaged in the manufacture, production, or wholesale in the United States of a domestic product that has the same physical characteristics and uses as the product for which a modification of an existing duty is sought