WASHINGTON — Just over two decades after lobbying unsuccessfully against the North American Free Trade Agreement, U.S. labor unions are again voicing strong reservations about a proposed major trade-liberalization deal.

At issue now is the Trans-Pacific Partnership, a measure expected to call for lowering or eliminating most trade barriers among the United States and 11 other Pacific Rim nations. The pact is still being hammered out in closed-door negotiations.

Union leaders and other critics say the proposed pact would prompt U.S. companies to funnel manufacturing jobs to lower-wage countries. Environmental and human rights groups also are voicing strong opposition.

It’s a familiar theme. After all, former presidential candidate Ross Perot warned NAFTA would create “a giant sucking sound” as jobs left the United States for Mexico.

So far, the economic consequences of NAFTA have seemed mixed — not as dire as Perot predicted nor as positive as former President Bill Clinton once forecast.

The current Pacific Rim free-trade debate pits many fellow Democrats against President Obama. Some Democratic critics have depicted the Pacific free-trade deal, and a companion trans-Atlantic one expected to follow, as “NAFTA on steroids.”

As with NAFTA, which linked the economies of the United States, Canada and Mexico and created what is now the world’s largest free-trade zone, the Pacific free-trade proposal generally has stronger support among Republicans in Congress than Democrats.

Up to now, Congress has shown little inclination to grant Obama trade promotion authority powers — called “fast track” — that allow only yes-or-no votes on trade agreements with no amendments permitted. Fast-track authority had been routinely granted by Congress to presidents in past major trade negotiations, including NAFTA.

Now, with Republicans in control of Congress, chances might seem to be stronger for free-trade legislation. But Gary Hufbauer, a senior fellow at the Peterson Institute for International Economics, predicted that Obama “is going to have a harder time bringing over Democrats than Clinton did back in that era.”