The White House is closely monitoring votes by the two largest rail unions set to take place Sunday on a labor deal that President Joe Biden personally negotiated, according to a person familiar with the matter. 

A no-vote by either union could lead to a walkout in early December that could have devastating effects on the US economy, unless Congress intervenes. The Biden administration is committed to taking every appropriate step to ensure the rail system delivers needed items, the person said without elaborating. 

Labor Secretary Marty Walsh brokered an agreement between union leaders and railroads in September to stave off a work stoppage, with Biden playing the role of closer. The deal helped stave off a massive supply chain disruption weeks before the midterm elections. 

Union members, however, still must approve the contracts, and conductors and engineers still angry over uncertain schedules and working short-handed during the pandemic have cast doubt over the ratification. 

A White House official, speaking on condition of anonymity, said the administration has made clear to all parties that a shutdown is unacceptable and urged both sides to work in good faith to avoid even the threat of a shutdown.

A unit of the Sheet Metal, Air, Rail and Transportation Workers that represents rail conductors and yardmasters and the Brotherhood of Locomotive Engineers, which comprise about half the rail workers, are voting this weekend, and the results are set to be announced Nov. 21. If the vote fails, it raises the prospects of a strike or walkout just before the holiday season.

Congress has the power to pass legislation to end a work stoppage and force workers back on the job, if the unions and railroads cannot come to terms. 

Such a scenario would pose a political dilemma for Biden, who has pledged to be the most pro-union president in history and also promised to unclog the nation’s supply chains that were snarled during the pandemic.