Whenever the United States, acting in pursuance of any of its rights or obligations under any trade agreement entered into pursuant to this chapter, section
1821 of this title, or section
1351 of this title, withdraws, suspends, or modifies any obligation with respect to the trade of any foreign country or instrumentality thereof, the President is authorized to proclaim increased duties or other import restrictions, to the extent, at such times, and for such periods as he deems necessary or appropriate, in order to exercise the rights or fulfill the obligations of the United States. No proclamation shall be made under this subsection increasing any existing duty to a rate more than 50 percent above the rate set forth in rate column numbered 2 of the Tariff Schedules of the United States, as in effect on January 1, 1975, or 20 percent ad valorem above the rate existing on January 1, 1975, whichever is higher.