(NewsNation) — With a potential rail strike less than two weeks away, U.S. Transportation Secretary Pete Buttigieg told NewsNation’s Leland Vittert that a shutdown “is not acceptable” but declined to say whether President Joe Biden’s administration would support rail workers if they strike.

On Monday, one of the biggest rail unions rejected its deal, joining three others that have failed to approve contracts over concerns about working conditions and a lack of paid sick time. If the parties can’t reach an agreement, rail workers could strike as soon as Dec. 5.

Buttigieg said the administration’s goal is to make sure a strike doesn’t happen.

“We’ve got to get to a solution that does not subject the American economy to the threat of a shutdown,” Buttigieg told Vittert. “We don’t have enough trucks or barges or ships in this country to make up for the rail network.”

Railroads haul about 40% of the nation’s freight each year and a shutdown would have ripple effects across the nation. Food manufacturers and retailers at the height of the holiday shopping season are bracing for disruptions.

Commuter rail systems, many of which rely on tracks owned by freight railroads, would also be impacted.

If the two sides can’t reach a deal, Congress could step in and force an agreement by imposing contract terms. Such an intervention would prevent a strike that could cost more than $2 billion a day, according to estimates by the Association of American Railroads.

The question is whether Democratic lawmakers are prepared to go against their political allies — unions.

When asked whether the Biden administration would back rail workers in the event of a strike, Buttigieg did not answer directly.

“I don’t want to get into a scenario over battle lines that haven’t fully been drawn yet,” he said, “but I will say is that we certainly believe in collective bargaining.”

Instead, the transportation secretary emphasized his support for the tentative agreement that was reached back in September.

“The tentative agreement was one that everybody moved in order to get done — the railroad companies and the unions,” Buttigieg said. “I think it was a good agreement.”

That agreement, which was proposed by a Biden-appointed Presidential Emergency Board, included 24% raises over a five-year period, $5,000 in bonuses and one additional paid leave day a year but some unions are pushing for more reliable schedules and better working conditions.

“Some people don’t even have rest days or days off,” Jeremy Ferguson, the president of Smart Transportation Division (SMART-TD), told NewsNation’s “Rush Hour” on Monday. “That makes life very difficult.”

SMART-TD, which has 28,000 members, is one of four unions that has yet to reach an agreement. Eight others have agreed to terms with the railroads but if just one union fails to reach a deal, all rail workers would be set to walk out.

Congress is expected to step in to settle the dispute as it did in 1991. Back then, lawmakers ended a national railroad strike less than 24 hours after it began.

The potential rail strikes comes a at a critical time for American travel infrastructure: the holiday season. AAA projects that more than 55 million Americans will travel 50 miles or more for Thanksgiving, and air travel is returning to its pre-pandemic levels.

The faster-than-expected recovery for airline demands has meant huge rates of cancellations and delays for consumers, which Buttigieg’s department has tried to crack down on. Just recently, the department announced more than $600 million has been returned to customers for refunds owed since 2020.

“I’ve pressed the airlines to take steps like step up their hiring or make their schedules more realistic,” Buttigieg said. “They have, and I think that’s one of the reasons we’re seeing better rates of service now than we saw over the summer.”

Buttigieg, the former mayor of South Bend, Indiana, who dropped out of the 2020 presidential race, has had his share of the limelight over the past two years serving in the Biden administration. He’s helped put an emphasis on projects funded by the bipartisan infrastructure law and has touted his work to enforce airline regulations to aid consumers.

“It’s the honor of a lifetime to be out here at this moment where we’re building infrastructure like we haven’t done in a half-century,” Buttigieg said. “It’s an incredible opportunity, and I’m pinching myself sometimes that I get to do this work.”