Acemoglu signed a public letter stating that U.S. manufacturing job losses are mostly not due to trade (Nov 2016):
manufacturing’s share of employment has been declining since
the 1970s and is mostly related to automation, not trade.
Barely ten months ago he published a paper saying that U.S. manufacturing job losses are mostly due to trade, particularly with China (Jan 2016):
Between 2000 and 2007, the economy gave back the considerable employment gains achieved during the 1990s, with a historic contraction in manufacturing employment being a prime contributor to the slump. We estimate that import competition from China, which surged after 2000, was a major force behind both recent reductions in US manufacturing employment and—through input-output linkages and other general equilibrium channels — weak overall US job growth. Our central estimates suggest job losses from rising Chinese import competition over 1999–2011 in the range of 2.0–2.4 million
For reference, this chart gives the employment numbers they're referring to: http://i2.cdn.turner.com/money/dam/assets/160329105836-manufacturing-jobs-decline-2000-780x439.jpg
Note that that virtually all employment losses occurred after 2000, in the same period that Acemoglu analyzed for his paper. The "since the 70s" part is just intentional obfuscation ("share of employment") by the public letter.
tl;dr According to Acemoglu, trade with China alone represented up to 48% of recent U.S. manufacturing losses. But most manufacturing losses are not related to trade.