“There are serious problems with this agreement,” Toomey told reporters, adding that it would be a “big mistake” to pass the U.S.-Mexico-Canada agreement in the coming weeks.

Toomey argued the GOP should have passed the deal while it still had control of the House instead of spending months negotiating with Speaker Nancy Pelosi on Democratic demands.

“It’s clearly moved way to the left, which is why you had a celebratory press conference by all the Democratic leadership in the House. … You have organized labor enthusiastically supporting this,” Toomey said. “When’s the last time they were enthusiastic about a free trade agreement?”

Toomey isn’t alone in his complaints that the deal may have moved too far to the left.

Sen. John Cornyn (R-Texas) has also raised concerns that U.S. Trade Representative Robert Lighthizer may have given “away the store” to Democrats and said on Tuesday that concessions to the AFL-CIO “concerns” him.

Sen. John Cornyn (R-Texas). | Zach Gibson/Getty Images

But Cornyn isn’t vowing to oppose the deal, unlike Toomey, who called it plainly a “step backward” from the existing NAFTA trade deal.

Toomey ticked off a litany of ostensible flaws in the agreement: There wouldn’t be adequate time to review it; it expires after 16 years; it increases costs for U.S. consumers; it’s “not going to do anything for economic growth”; and it is “a complete capitulation to Democratic demands on intellectual property for biologics.”

“This would be a terrible new standard” for future trade agreements, Toomey said.

Marianne LeVine contributed to this report.