Last June, Portman was one of 60 senators to provide the Obama administration with the authority to complete negotiations on the pact, which meant Congress could only approve or reject the final pact but not change it.
But Senate Majority Leader Mitch McConnell, R-Ky., has ruled out a Senate vote this year on the deal in part because it is unpopular in many states, including Ohio.
Portman will face the winner of the Democratic primary this spring between former Gov. Ted Strickland and Cincinnati city councilman P.G. Sittenfeld.
Democrats accused Portman of attempting to line his position up with public opinion in Ohio.
Daniel van Hoogstraten, a spokesman for the Ohio Democratic Party, called it “duplicitous double talk” and added, “he’ll say anything to help himself.”
Dale Butland, a Sittenfeld spokesman, accused Portman of being “multiple choice” on trade, saying “first he was for” the trade deal and “now he’s against it.”
Both Strickland and Sittenfeld have staked out positions against the Pacific agreement.
In his statement, Portman said, “From currency manipulation, to rules of origin for automobiles, to protection for US biologics — we can do better. And we need to do better for the sake of American workers who are depending on the Administration to give them a level playing field.”
U.S. lawmakers have long complained that China engages in currency manipulation to make their exports more financially competitive. But China is not part of the new agreement.
Instead, the 12 nations are the United States, Canada, Mexico, Australia, New Zealand, Vietnam, Singapore, Brunei, Japan, Chile, Peru and Malaysia.
Obama, in a statement issued after the pact was signed, said current global trade rules “too often undermine our values and put our workers and businesses at a disadvantage.”
The new agreement, Obama said, “will change that. It eliminates more than 18,000 taxes that various countries put on ‘Made in America’ products,” adding it “allows America – and not countries like China – to write the rules of the road in the 21st century, which is especially important in a region as dynamic as the Asia-Pacific.”