Buffenbarger said his comments applied to the congressional races. In the presidential race, his union has already endorsed Hillary Rodham Clinton. It's unclear where Clinton will come down on the trade agreement, but Buffenbarger said the endorsement stands.
Clinton's chief rival for the Democratic nomination, Sen. Bernie Sanders, I-Vt., moved quickly to voice his opposition.
"Wall Street and other big corporations have won again. In the Senate, I will do all that I can to defeat the #TPP agreement," Sanders tweeted.
Another Democratic candidate, former Maryland Gov. Martin O'Malley, has been highly critical of the trade pact in recent months.
The TPP is designed to encourage trade among the United States, Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam. The pact would reduce tariffs in the participating nations in a bid to open markets.
Democratic lawmakers representing major manufacturing districts voiced skepticism that the pact would help their constituents.
"American workers should be skeptical of the specifics of this new trade agreement. Past trade deals like NAFTA have hollowed out America's manufacturing base and shipped thousands of American jobs overseas," said Rep. Dan Kildee, D-Mich.
As some of the expected opponents quickly objected, others needed to advance the deal voiced some caution. Members of the House Agriculture Committee have said they're concerned about a lack of access for rice farmers and the dairy industry.
"While I am encouraged to hear that U.S. livestock products such as beef and pork will see significant gains in market access, it will take a coalition of many to move TPP over the coming months," said committee Chairman Michael Conaway, R-Texas. "At this time, I am skeptical that these concerns were sufficiently addressed but will remain open-minded."
The Associated Press