The threat of a rail strike on Friday has already begun rippling through the U.S. economy, as farmers, businesses and commuters start to feel the impact of a potential transit shutdown even before it happens.

While top negotiators huddled in Washington late Wednesday to try to hammer out an agreement, industry groups increasingly warned of severe disruptions to America's already fragile supply chains. Scheduled shipments of ammonia, fertilizer and other chemicals for agriculture are being pulled this week, because those products cannot be stranded in transit should negotiations fail, farm groups said. Ethanol prices moved markedly higher this week on the threat of a strike, according to S&P Global. U.S. railroads are also prepared to stop shipping crops as soon as Thursday, the Consumer Brands Association reported.

On the passenger rail side, Amtrak is canceling all long-distance trains starting Thursday, although most trains in the Northeast won't be affected, Amtrak said.

The emerging economic impact is putting enormous new strain on leaders on Capitol Hill and in the White House who are trying to end the standoff between rail carriers and workers. Labor Secretary Marty Walsh hosted emergency meetings with the rail carriers and unions on Wednesday at the department's D.C. offices, but no sign of a deal has emerged. As of 6 p.m. Eastern, negotiators had been in closed-door talks for nine hours and had ordered in Italian for dinner.

Meanwhile, Republican leaders in Congress pushed legislation Wednesday to force the labor unions and management to accept the contract recommendations of a presidential board. Democrats rejected that proposal.

Eight unions have reached tentative agreements with the carriers based on the board's recommendations, leaving four unions without a deal in place, including two of the largest and most politically powerful.

But hopes for quick resolution were further clouded Wednesday by one local chapter's rejection of the deal brokered by their leadership. A chapter of the International Association of Machinists authorized a strike to take place later in September, hinting at broader rancor among union workers about the agreements being forced between union leadership and management.

There also was some forward momentum on Wednesday, as the members of two unions representing railway car inspectors and clerical workers became the first to vote to ratify their contracts negotiated by the president's appointed board.

Negotiators face a deadline of 12:01 a.m. Eastern on Friday to avert the freight shutdown. Tens of thousands of workers would be part of a strike, if authorized.

Labor leaders have said they think the rail carriers are prematurely ceasing operations as a tactic to increase pressure on negotiators to reach a deal.

"If rail shuts down, our entire agricultural system shuts down," Sen. Deb Fischer, R-Neb., said in a speech on the Senate floor, emphasizing that her state's farmers were counting on the upcoming harvest season. "The stakes of these rail negotiations could not be higher."

The Association of American Railroads has estimated that failure to reach an agreement could cost the U.S. economy as much as $2 billion per day in lost economic output.

Aides to President Joe Biden have sought to resolve the conflict between the rail carriers and unions to avert the possibility of one of the most disruptive strikes in recent U.S. history. The stakes are high for the Biden administration, which is desperate to ensure that America's trains keep running but does not want to undermine the demands of union workers.

The administration has already faced criticism over its handling of the nation's transportation infrastructure, which was racked last year by supply chain snarls and this year by a spike in cancellations and delays at the nation's airports. Some administration officials fear squandering the Biden economic victories of August that have helped boost Democrats' poll numbers.

Walsh, Transportation Secretary Pete Buttigieg and Agriculture Secretary Tom Vilsack have been in frequent communication with both sides of the negotiations, and Biden has personally called the unions and carriers to urge a deal.

The remaining issues of dispute include points-based attendance policies for conductors and engineers that penalize them for going to routine doctor visits or responding to family medical emergencies. The two largest railway unions have said their members would not ratify a contract that ignores this issue, and so far the railroads have not made any moves on the matter. It is unclear how Walsh or the administration plans to break the impasse.

The increasing desperation started new fights in Congress to resolve the matter. Sens. Roger Wicker, R-Miss., and Richard Burr, R-N.C., on Wednesday afternoon attempted to advance legislation to force both sides to accept the contract recommendations made last month by a nonpartisan panel appointed by the president. The unions have rejected those recommendations because they do not address workers' fury over company penalties for missing time because of illness or family emergencies.

Sen. Bernie Sanders, I-Vt., on Wednesday blocked the GOP attempt to pass the measure via "unanimous consent," which requires the support of all 100 senators.

The lawmakers' next move is unclear. On Tuesday, Senate Minority Leader Mitch McConnell, R-Ky., said that he supports adoption of the board's recommendations and called on the president to do the same. House Speaker Nancy Pelosi, D-Calif., told reporters on Wednesday that she hopes negotiations succeed so there is no need for Congress to intervene.

Democratic lawmakers have largely left it to the Biden administration to reach a deal, and it's not clear whether they would support the GOP's legislation, against the wishes of labor unions, should the impasse reach that point.

The congressional jockeying comes as a new poll suggests that most workers in one of the biggest railroad unions are prepared to reject the deal under consideration. The poll by SMART-TED found that 78 percent of workers would reject the proposed settlement.

"I know for sure with COVID out there nobody is even testing themselves because they don't want to lose points," said Jordan Boone, 41, a BNSF conductor in Galesburg, Ill., and member of SMART-TED. "I have five kids, and I've always missed the kids' soccer and baseball games and cheerleading, but the new attendance policies make it impossible to go to anything."

Democrats are highly unlikely to approve legislation mandating that workers accept the contract recommendations without changes to time-off policy, said Larry Cohen, a labor leader and former president of the Communications Workers of America.

"Democrats are not going to impose these contracts without dealing with the issue of workers' working lives," Cohen said. "Republicans are viciously against collective bargaining, but carriers are going to have to respect people's lives and there's going to have to be respect for these workers. They're not getting a settlement without it."

Still, political pressure is mounting on Democrats to agree to end the standoff. White House aides have in recent days examined the potentially drastic impact on the nation's drinking water and energy supplies that could come from a shutdown.

"If it's a day or two, it may not be that big a deal. If this went on for a week or two weeks, you'd see shortages of all sorts of things," said Dean Baker, a White House ally and co-founder of the Center for Economic and Policy Research, a left-leaning think tank. "You're going to have erratic shortfalls - they're trying hard to do this but it's really hard to do on the fly. It's not like they've been planning this for years."

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The Washington Post's Luz Lazo, Laura Reiley and Jaclyn Peiser contributed to this report.