(Reuters) - Fifteen Asia-Pacific economies are set to conclude talks on Sunday and sign what could become the world’s largest free trade agreement, covering nearly a third of the global population and about 30% of its global gross domestic product.The Regional Comprehensive Economic Partnership (RCEP), which could be approved at the end of a four-day ASEAN summit in Hanoi, will progressively lower tariffs and aims to counter protectionism, boost investment and allow freer movement of goods within the region. A U.S.-China trade war and U.S. President Donald Trump’s “America First” retreat from predecessor Barack Obama’s “pivot” towards Asia has given impetus to complete RCEP, which is widely seen as Beijing’s chance to set the regional trade agenda in Washington’s absence. The U.S. election win by Democrat Joe Biden, however, could challenge that, with the former vice president signalling a return to stronger U.S. multilateralism.