WASHINGTON (REUTERS) - United States Senate Democrats delivered a major blow to President Barack Obama's trade agenda on Tuesday, blocking debate on a bill that would have smoothed the path for a Pacific trade pact.

The stunning outcome cast doubt on legislation that is key to the Obama administration's ability to complete the 12-nation Trans-Pacific Partnership (TPP), which includes Singapore as an original signatory, a central plank of its pivot to Asia. It would give lawmakers the right to set negotiating objectives but restrict them to a yes-or-no vote on trade deals such as the 12-nation TPP, a potential legacy-defining achievement for Mr Obama.

"What we just saw here is pretty shocking," said Senate Majority Leader Mitch McConnell, a Republican, after the Senate voted 52-45 - eight short of the 60 votes needed - to pave the way for debate on "fast-track" trade authority for Mr Obama.

The legislation would allow a quick decision on granting the President so-called fast track authority to move trade deals quickly through Congress. The vote marked a big victory for Senate Democratic leader Harry Reid, an outspoken opponent of fast-track.

The failure to garner the necessary votes came after key pro-trade Democrats, including Senator Ron Wyden of Oregon, announced they would vote no on the procedural vote because the measure lacked some trade protections.

"This is an issue worthy of our consideration and yet today we have voted to not even consider it," said Sen McConnell.

Only one Democrat, Senator Tom Carper of Delaware, backed the measure, despite a White House campaign blitz to win Democratic support.

Senate Republicans stuck together in voting to let the Bill pass its first test. However, Sen McConnell at the last minute switched his vote to "no" in a procedural move that lets him bring the Bill to a vote again in the future.

Mr Obama held a"constructive session" with 10 Democratic senators on Tuesday after a vote in which they blocked the fast-track trade Bill needed to finalize a Pacific trade pact, the White House said in a statement.

"Members in attendance reiterated their support for TPA (Trade Promotion Authority) legislation that will pave the way for high-standard trade agreements that support good American jobs, protect our workers and environment, and ensure that the United States, and not countries like China, write the rules for the global economy," the White House said.

Failure of the Bill sends a worrying signal about the level of support for fast track, which unions, environmental and consumer groups strongly oppose, along with some conservatives. It may also reflect congressional maneuvering tactics after Sen Reid demanded that the Senate first consider the imminent expiration of federal transportation construction projects and a domestic surveillance programme.

The White House strongly opposes the measure. A source familiar with negotiations in the Senate said one option would be to bundle the four bills together but take out the currency provision. That could be voted on separately, said the source, who asked not to be identified and stressed talks were continuing.

Democrats have also complained that the TPA measure falls short on labour and environmental protections.

As Democratic support for the measure melted away, Senate Finance Committee Chairman Orrin Hatch, a Republican who was a major force for fast track, declared to reporters that the measure "may be dead".

But following the vote, Democrats predicted it would come back. "I don't think today's vote is a death knell for TPA, but it is a very strong warning shot to the majority leader ... that without worker protections, without enforcement provisions, they will likely not move forward," Democratic Senator Chris Coons told reporters.

Negotiations on the TPP are nearly complete, but trading partners have said they want to see fast-track legislation enacted before finalizing the pact, which will stretch from Japan to Chile.

Republicans could now work on the transportation and surveillance bills and then try again on trade at some time in the future. The TPP would create a free trade zone covering 40 per cent 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.

Although the administration pointed to research saying export-related jobs pay up to 18 per cent more than other jobs, other studies showed that increased competition from imports has cut wages and caused job losses in US manufacturing.