By Krista Hughes and Richard Cowan
WASHINGTON (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.
"This is going to be an old fashioned cliffhanger," said a senior Senate Democratic aide.
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.
Senate supporters acknowledged they would have to work right up to the mid-afternoon vote to secure enough support to score an initial victory.
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.
Senate Majority Leader Mitch McConnell, hoping to shore up support, reminded his fellow senators that Tuesday's vote simply would pave the way for debating fast-track legislation.
It "is not a vote to approve or disapprove Trade Promotion Authority," McConnell said.
He said the fast-track bill would be paired with a measure to provide training to workers who lose their jobs as a result of any trade deals. While many Republicans oppose that legislation, Democrats are demanding its inclusion.
Furthermore, McConnell said other trade measures Democrats seek could be debated as amendments to the fast-track bill.
But Senate Democratic leader Harry Reid warned that without further trade protections being incorporated into the fast-track bill, "it's going to be very difficult to get to the guts of this, the bills we reported out of committee."
If McConnell fails to obtain the 60 votes needed on this first procedural vote, he is expected to take steps that would allow him to try again.
"As you know, one of the things he (McConnell) could do is to move to reconsider at a later time," said Senator John Cornyn, the Senate's No. 2 Republican.
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.
McConnell needs the support of at least six Democrats to move the bill forward. At the same time, it was unclear whether all 54 Republicans would vote to clear the way for simply debating the bill.
If the test vote succeeds, the Senate could start weighing amendments to the bill including tough rules against currency manipulation in trade deals, which the Obama administration opposes, and human trafficking.
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.
(Additional reporting by Susan Cornwell; Editing by Steve Orlofsky)