In Vietnam this week, President Barack Obama spoke in support of the free-trade agreement called the Trans-Pacific Partnership.

The free-trade deal is among the United States and 11 other nations around the Pacific Ocean.

On Monday, he said he was "confident" that the deal, known as the TPP, will pass in the U.S. Congress. Vietnam is one of the countries included in the TPP.

But members of the U.S. Senate, which approves trade agreements, are not so sure.

Republican Senator Lindsey Graham of South Carolina said the president was "overly optimistic" about passage of the deal. "There's been a sourness on trade" in Congress, he added.

Democrats in the Senate also have voiced concerns. Chuck Schumer of New York called passing the TPP a "tough lift," or a difficult effort.

Dick Durbin of Illinois pointed to the lack of support for the agreement among the candidates for president.

"When you have the three leading presidential candidates of both parties opposing TPP, it's an indication that political sentiment is not in favor of the agreement," said Durbin.

Democratic Party candidates Hillary Clinton and Bernie Sanders, and Republican Party candidate Donald Trump, have said they oppose the TPP trade deal.

Some labor unions, environment groups and human-rights groups also strongly oppose the deal.

Details Still an Issue for Some Supporters

The TPP was created to reduce import taxes, such as tariffs, which are considered barriers to trade. It also puts in place rules for trade and enforcement. Partners include Japan, Canada, Australia, Mexico and Vietnam. The 12 countries that signed the agreement make up about 40 percent of the world's economy.

American business groups support the agreement.

Democratic Senator Mark Warner of Virginia supports the TPP.

"There are winners and losers in every trade deal," he said. Many problems linked to trade with China, such as job losses, have already taken place, he said. The TPP, he noted, does not deal with China trade.

Supporters of the TPP want Congress to vote on the agreement after the November elections.

That is a possibility, according to Republican Senator Orrin Hatch of Utah. But Hatch is unsure about the TPP. He said, "I think Republicans do want to support it, but there need to be some changes."

Hatch is not happy about the length of time U.S. companies would keep exclusive rights to their drug and biological products. These include drugs and genetically engineered products.

"The most problematic area is the data exclusivity provision of only five years," the Senator said. "We need to solve some of these problems, but I think they are solvable," he added.

Partner nations signed the TPP in February in New Zealand after seven years of negotiations.

I'm Mario Ritter.

Michael Bowman reported this story for VOA News. Mario Ritter adapted it for Learning English. Kathleen Struck was the editor.