(Repeats without changes to widen readership)
* Fast-track legislation faces key congressional hurdle
* Needs to clear 60-vote threshold to proceed
By Krista Hughes and Richard Cowan
WASHINGTON, May 12 (Reuters) - The Pacific trade agreement faces its first test in the U.S. Senate on Tuesday in a knife-edge vote that may hold the key to President Barack Obama's diplomatic pivot to Asia.
The Senate vote is one of a likely series of congressional hurdles to be overcome that will hinge on the support of a handful of Democrats. The White House has launched a campaign blitz directed at them in support of granting the president authority to speed trade deals through Congress.
Fast-track legislation gives lawmakers the right to set negotiating objectives but restricts them to a yes-or-no vote on trade deals such as the 12-nation Trans-Pacific Partnership, or TPP, a potential legacy-defining achievement for Obama.
"A loss would not be fatal ... but it's very important to keep the ball rolling forward," said Fred Bergsten, founding director of the Peterson Institute for International Economics.
The TPP would create a free trade zone covering 40 percent of the world economy - making it the biggest trade deal since the North American Free Trade Agreement liberalized trade 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.
"It's incredibly important for American workers that we pass this bill," said Senate Majority Leader Mitch McConnell, a Republican, on Monday. Without approval, he said, foreign companies would have easier access to U.S. markets than U.S. businesses would have to theirs.
Senate Democratic leader Harry Reid, a fast-track opponent, wants debate on the imminent expiration of federal transportation construction projects and a domestic surveillance program before Congress takes up fast track.
McConnell will have to find 60 votes in the 100-member Senate to overcome procedural obstacles before work can formally begin on the fast-track bill, which trading partners have said they want to see enacted before finalizing the TPP.
That requires support from at least six Democrats. But late on Monday, No. 2 Senate Democrat Dick Durbin questioned whether McConnell could find enough votes to bring the measure to the floor.
If the test vote succeeds, the Senate could start considering amendments to the bill including tough rules against currency manipulation in trade deals, which the Obama administration opposes, and human trafficking.
If it fails, Republicans could work on the transportation and surveillance bills and then try again, or push ahead on trade without limiting speaking time on the bill, virtually guaranteeing a lengthy debate.
Failure would send a worrying signal about the level of support for fast track, which unions, environmental and consumer groups strongly oppose, as do some conservatives.
"If the cloture vote fails, I think passage becomes less likely," said United States Business and Industry Council President Kevin Kearns, a fast-track critic.
The White House is pushing Democrats to support the TPP and fast track. Seven of 12 Democrats backed the bill during Senate committee debate.
"This is really an acid test on whether members of Congress believe in looking outward or looking inward," said Bill Lane, Global Governmental Affairs director for Caterpillar Inc , one of many companies pushing for the deal.
(Reporting by Krista Hughes and Richard Cowan; Editing by Steve Orlofsky)