US President Joe Biden signed into law on Friday a rare congressional intervention forcing rail unions to accept a salary agreement, avoiding a strike that has been called for, for quite some time. 

In a brief ceremony at the White House, Biden signed the bill just a week before unions who had rejected the deal were set to strike, threatening critical supply chains across the world's largest economy.

The agreement provides a significant wage increase, but four of the 12 unions involved refused to accept it because there was no agreement on providing workers with paid sick leave. To settle railroad disputes, Congress used an underutilized power. As he signed the bill, Biden said Congress had "avoided what, without a doubt, would have been an economic catastrophe."

"Without freight rail, many of the US industries would literally have shut down," Biden said, adding that his advisors feared the loss of three-quarters of a million jobs within two weeks if the strike had gone ahead.

The episode is politically awkward for Biden, who frequently boasts about his pro-labor credentials. He was scheduled to meet with electrical union members in Boston later Friday.

Trade unions are an important part of his electoral coalition, and he frequently refers to himself as a lifelong union supporter and the "most pro-union president" in history.

The emergency bill signing has harmed Biden's brand, with some on the left accusing him of betraying his principles. One union leader described the situation as "horrible" after the Senate ruled decisively in favor of rail management.

Senators, according to the Brotherhood of Railroad Signalmen, have "demonstrated their support for the corporate class."

Read next: Congress must intervene to prevent 'devastating' US rail strike: Biden

Earlier, the largest rail union in the United States voted against a provisional September contract deal reached, meaning a strike is more likely to take place in the coming weeks, severely damaging the US economy.

Train and engine service members of the transportation divisions of the International Association of Sheet Metal, Air, Rail, and Transportation Workers (SMART-TD) voted with a slight majority to reject the deal.

Meanwhile, members of the Brotherhood of Locomotive Engineers and Trainmen (BLET) voted in favor of ratifying the agreement, with SMART-TD yardmasters voting in favor of ratifying their national agreement, the unions said.

According to CNN, unless a potential freight rail strike due to failed labor negotiations is averted, this could cost the US economy up to $1 billion in the first week of the strike.

New analysis from the Anderson Economic Group (AEG), the initial three days of a possible strike could cost at least $250 million.

"Economic impacts caused by a national railroad strike include lost wages for the industry’s workers and production slowdowns due to non-delivery of critical components in some vulnerable industries," the AEG analysis said as quoted in the report.