Sen. Robt Portman (R-Ohio) is in a tight re-election race where trade and its impact on jobs in the state is a major issue. (Photo by Ty Wright/For The Washington Post)

Sen. Rob Portman, a former U.S trade ambassador, announced Thursday that he opposes a sweeping 12-nation Pacific Rim trade agreement, dealing a setback to a deal that is seen as a key part of President Obama’s economic legacy.

The Ohio Republican is facing a difficult re-election campaign against Ted Strickland, an anti-trade former Democratic governor, in a state that has seen a steep decline in manufacturing as a result of companies moving operations overseas. The announcement is a significant but not fatal blow to the Trans Pacific Partnership (TPP) agreement, which is protected by fast-track rules that ensure it cannot be filibustered in the Senate.

“I cannot support the TPP in its current form because it doesn’t provide that level playing field,” Portman said in a statement. “I will continue to urge the Obama administration to support American workers and address these issues before any vote on the TPP agreement.”

Portman, who served as the top U.S. trade official under President George W. Bush, was seen as a potential ally for the Obama administration. Last year he voted for legislation to grant Obama fast-track trade negotiating authority. That bill, considered a bellwether of support for the trade agreement itself, passed on a 62 to 37 vote in May.

Strickland used Portman’s announcement as an opportunity to knock his past support for trade deals.

“The difference between Senator Portman and myself when it comes to trade is clear: he voted for 8 trade deals and I opposed them,” he said in a statement. “He voted to make permanent most favored nation status for China, and I opposed it.”

The TPP agreement was signed Wednesday in a ceremony in New Zealand but has not yet been transmitted to Congress for official consideration. Support for the agreement has waned in recent months and Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday that he does not expect to consider the deal before the November election.

[U.S., 11 nations formally sign largest regional trade deal in history]

“It’s pretty obvious to anybody who will state the obvious that with both the Democratic candidates for president opposed to the deal and a number of the presidential candidates of our party opposed to the deal, it’s my advice that we not pursue that, certainly before the election,” McConnell said.

Portman left open the possibility that he could change his position and support the deal if changes are made to better protect U.S. workers. He announced last year that he was skeptical of the deal for failing to prevent countries like Vietnam and Japan from artificially devaluing or otherwise manipulating the value of their currency. He drew criticism from his Republican colleagues for supporting a currency amendment to the fast-track bill.

The statement also listed concerns with the complex rules of origin that are used to determine how countries can source parts for major exports like automobiles. Many U.S. companies worry that the TPP makes it too easy for countries like Japan to undercut U.S. automakers by buying cheap parts from China to build inexpensive cars destined for U.S. dealerships.

The fight over rules of origin has plagued the deal throughout negotiations as have concerns that it doesn’t do enough to protect U.S drug innovations. U.S. pharmaceutical companies wanted data and research on complex biologic drugs to be protected for 12 years, a measure Portman and other Republicans supported. The final deal cut the protections to five years.

[In a setback for Obama, negotiators fail to wrap up Pacific trade pact]

The Obama administration has hailed the TPP as the most extensive trade expansion in a generation. Supporters of the agreement originally hoped the bipartisan vote on the fast track bill was a signal that the trade deal would have an easy path in Congress despite election year politics.

Hopes fizzled in December when McConnell announced in an interview with The Washington Post that he thought it would be a mistake for Obama to try to pass the deal in an election year. Nearly every presidential candidate in both parties oppose the deal as do many House Democrats.

[McConnell warns that trade deal can’t pass Congress before 2016 elections]

Fast-track legislation lays out a strict timeline that requires Congress to vote on the legislation within 90 days of the signed agreement being transmitted for their consideration. Transmission could take time and there is a chance that Obama could work with Congress to slow-walk the process long enough for deal to come up after the November election.

McConnell and House Speaker Paul D. Ryan (R-Wisc.) are both in favor of expanding U.S. trade and Ryan is credited with carefully persuading House Republicans to back fast track while he was chairman of the House Ways and Means Committee.

“The speaker’s a free trader. I’m a free trader, and obviously, the president is as well,” McConnell said earlier this week. “There are a number of flaws here. We’re gonna keep on talking about it and see if there’s a way forward.”