According to news reports, a cloture motion to cut off a filibuster and proceed to debate fell short of the 60 votes necessary to pass. Sen. Tom Carper, of Delaware, was the only Democrat to vote yes.

Civil society groups lauded Tuesday's outcome and what it could mean for future trade votes.

"The Fast Track train went off the rails today," cheered Lori Wallach, director of Public Citizen’s Global Trade Watch division. "The U.S. Senate vote was supposed to generate momentum for Fast Track in the U.S. House of Representatives, where it's in deep trouble, with almost every House Democrats and a significant bloc of GOP opposing it."

Still, now is not the time for the grassroots to become complacent, warned Democracy for America executive director Charles Chamberlain in a statement released just after the vote.

"While we celebrate today's failed Fast Track vote for the job-killing Trans-Pacific Partnership, the hundreds of thousands of grassroots activists who have united behind Senators Warren, Brown and Sanders to defeat the TPP will not rest until it's dead, buried, and covered with six-inches of concrete," Chamberlain said. "Today, the army of corporate executives and industry lobbyists who wrote the Trans-Pacific Partnership by and for themselves failed to secure support for the Fast Track legislation they know they need to ram their bad trade deal through Congress."

However, he added, "We know the forces pushing the job-killing TPP won't stop here, and they should know, neither will we."

And Sen. Bernie Sanders (I-Vt.), who has been a vocal opponent of Fast Track and gave a stirring speech on the Senate floor prior to the vote on Tuesday, added: "The Senate vote today was an important first victory in what will be a long battle."

"Today was a good step forward," he said, "but much more needs to be done."

Amid convoluted political machinations and ever-mounting progressive opposition, the U.S. Senate is expected to vote Tuesday on a motion to move forward with Fast Track authority, which would for six years help ram corporate-friendly trade deals like the Trans-Pacific Partnership (TPP) through Congress.

The vote is likely to be very close—"a cliffhanger," according to Politico; "a knife-edge vote," says Reuters; "a squeaker," as per the Washington Post's Dana Milbank.

Proponents in the Senate must secure 60 votes on Tuesday to begin debate on Fast Track, or Trade Promotion Authority. While the vote is procedural, the Wall Street Journal notes that it "could indicate where members stand" on the authority, so fervently sought by President Barack Obama and his Cabinet, which would pave the way for passage of the 12-nation TPP.

Reuters notes, "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."

"Even if the free-traders get the required 60 votes, supporters won’t have momentum going into a vote in the House, where the legislation faces a tougher slog," Milbank added.

As the Washington Post reports, the "vast majority of the 245 House Republicans are expected to support the president, leaving White House officials to find possibly 25 to 30 Democratic votes. According to one Democratic estimate provided Monday, there are just 17 House Democrats so far supporting [Fast Track]."

According to news reports, Senate Minority Leader Harry Reid has pledged to block the trade bill until senators agree to a way forward on highway and surveillance bills, both of which face end-of-the-month deadlines.

In addition, Sen. Ron Wyden (D-Ore.), the top Democrat on the Senate Finance Committee who negotiated the details of the Fast Track bill introduced last month, has told his colleagues that he will not support starting the debate unless Republicans promise to also move forward with two other trade measures: a customs enforcement bill that would also crack down on currency manipulation and the African Growth and Opportunity Act that provides trade preferences to sub-Saharan African countries.

According to Politico, "Wyden’s vote is being watched closely by both Democrats and Republicans as the pivotal yes or no that will determine whether work on the trade bill can begin now or must wait until June."

"If Wyden votes no, he may take enough pro-trade Democrats with him to sink Tuesday’s vote," write Politico journalists Burgess Everett and Manu Raju. "That possibility has boosted the anti-trade faction of the Democratic Party, which is now predicting that without [Senate Majority Leader Mitch] McConnell’s intervention the vote on Tuesday will fail."

The Post reports that Minority Whip Richard J. Durbin (D-Ill.) said he had "no indication" that Republican leaders would agree to combine the various bills in such a way that would pass muster with enough Democrats to proceed.

"I would say, at this point, most Democrats are inclined to vote no unless they know what they're voting for," he said.

However, "McConnell faces a delicate calculus," the Wall Street Journal adds, because placating Senate Democrats by including one or all of the additional provisions "could create opposition among Republicans who support the bill, thus making it harder to line up enough support to ensure passage."

Of course, such political horse-trading does little to address the fundamental problems with Fast Track or the dangerous trade deals the authority is designed to promote, which progressive groups were quick to underscore on Tuesday.

In a statement, Evan Greer of the digital rights group Fight for the Future declared: "Today, every United States Senator has a decision to make: will they stand with the overwhelming majority of their constituents who oppose the dangerous secrecy inherent in the Fast Track / Trade Promotion Authority process, or will they bow down to the White House and the incumbent industries who have been spending millions lobbying to protect their power?"

Labor groups also escalated their rhetoric. "America is in an abusive relationship with trade-obsessed politicians and corporations," Leo W. Gerard, United Steelworkers international president, wrote in a blog post published Tuesday. "Despite their long history of battering the U.S. middle class with bad trade deal after bad trade deal, these lawmakers and CEOs contend workers should believe that their new proposal, the Trans-Pacific Partnership (TPP), will be different."

And in an interview with Greg Sargent of the Washington Post published Monday, Sen. Elizabeth Warren continued her verbal sparring with Obama, warning that passing Fast Track would have long-term implications. Because Trade Promotion Authority would apply for six years, for example, the next president could potentially "negotiate a trade deal that undercuts Dodd Frank," Warren said, referring to the financial reform bill passed in the wake of the Great Recession. 

As for the TPP's controversial Investor-State Dispute Settlement provision, which she has repeatedly criticized, Warren said it "imposes a financial penalty, which has caused countries to change their regulations…[ISDS mechanisms] never had the authority to override regulations. What they had was the authority to impose a monetary penalty directly against the government and its taxpayers. That’s the point at which governments have backed up and said, 'we can’t afford this, we’ll just change the law'."

Meanwhile, in the House, environmental and public health groups are condemning Rep. Earl Blumenauer (D-Ore.) for coming out in support of Fast Track in a joint op-ed with Wyden published last month in The Oregonian. On Tuesday, Food & Water Watch, Friends of the Earth, and Greenpeace began running television advertisements in Portland, Ore. and Washington, D.C. blasting Blumenauer as "wrong on trade."

The groups point to provisions in the TPP and other so-called "free trade" deals that would require the Department of Energy to automatically approve natural gas exports to countries included in the pact—which they say would lead to more fracking around the U.S.

"Simply put: a vote for Fast Track is a vote to accelerate the inherently environmentally dangerous practice of fracking for natural gas," said Wenonah Hauter, executive director of Food & Water Watch. In helping pass the measure through the House Ways and Means Committee, she continued, "Rep. Blumenauer voted to give foreign oil and gas companies powerful new trade litigation tools to go after the growing grassroots movement to ban fracking in statehouses, county seats and municipalities across the country."