U.S. senators backed tough new rules against currency manipulation ahead of an important test vote that holds the key to completing a Pacific Rim trade deal, the backbone of the Obama administration's pivot to Asia. The Senate voted 78 to 20 to allow the United States to slap import duties on goods from countries found to be artificially lowering the value of their currencies to gain an export advantage, despite objections from the White House that the measure would breach international trade laws. Still, a deal to separate the currency rules from the fast-track authority legislation reduces the chances that the bill will interfere with the passing of trade promotion authority (TPA.) A vote on granting the White House fast-track authority on trade deals is seen in the coming hours and is expected to face a tougher fight in the House of Representatives than the Senate. "I am hopeful that this currency bill, as well as the customs enforcement bill in general, will end up on the president's desk," said Charles Schumer, the number three Democrat in the Senate, who authored the import duty rules. "They may well find that they need currency to move along TPA to get anything done in the House of Representatives." Just two days earlier, Obama's own Democrats defied him and blocked consideration of the authority needed to complete the Trans-Pacific Partnership (TPP) trade accord with 11 other Pacific Rim countries ranging from Chile to Japan. The TPP pact, a key part of Obama's strategy to counter China's rising economic and diplomatic clout in Asia, would be the biggest trade deal since the North American Free Trade Agreement (NAFTA) liberalized business between the United States, Canada and Mexico. More than two decades later, that pact is blamed by many on the left for factory closures and job losses and has soured sentiment toward the TPP. Under fast track, the U.S. Congress can approve or reject the TPP deal, but not amend its contents, reassuring trading partners the agreement will not be picked apart by lawmakers. On this second try, enough Democrats are expected to support the legislation after they succeeded in scheduling a separate vote on a trade enforcement bill containing the currency rules. In the previous two days, Obama talked with almost a dozen senators and called others in to meet at the White House, a senior administration official said late on Wednesday. Senate passage does not guarantee the House of Representatives will embrace the currency rules. Furthermore, Obama could veto such a bill if it reached his desk, claiming it would threaten relations with trading partners. The Senate also passed a bill extending duty-free access to U.S. markets for African and other developing nations, a measure that is set to boost agricultural imports from the continent. Assuming Senate Majority Leader Mitch McConnell musters the 60 votes needed in the 100-member Senate to limit debate on whether to bring the fast-track trade bill to the floor, senators are expected to spend at least part of next week debating amendments before voting on passage. Left-wing Democrats want tougher controls on foreign labor and environmental standards, which they see as necessary to create a level playing field in any free-trade agreement the United States enters. Some conservative Republicans, meanwhile, could end up voting against fast-track next week because they do not want to give Obama any new powers. Trade is a hot-button issue with many Democrats in the United States, as labor unions and environmentalists - two of their important political supporters - are actively trying to kill fast-track. Obama's aggressive defense of fast track has put him at odds with the left wing of the Democratic Party, pitting him against Senator Elizabeth Warren, a leading liberal voice. (Reuters)