The auto workers’ strike against Detroit’s Big Three went into its fourth day on Monday with no signs of an early breakthrough and against the threat that the walkout could soon spread.

Treasury Secretary Janet Yellen said she is hoping for a quick resolution, and that it is too soon to gauge the impact of the strike.

“It’s premature to be making forecasts about what it means for the economy. It would depend on how long the strike lasts and who would be affected by it,” she said on CNBC.

Yellen said labor activism this year — strikes by Hollywood writers and actors, by workers at about 150 Starbucks locations and walkouts that were narrowly averted at United Parcel Service and West Coast ports — has been driven by a strong labor market and high demand for workers.

In a sign of the potential economic and political of a long strike, President Joe Biden is sending two top administration officials to Detroit this week to meet with both sides. Biden has sided with the UAW in brief public comments, saying that the automakers have not fairly shared their record profits with workers.

An administration official said Monday that acting Labor Secretary Julie Su and senior aide Gene Sperling will not serve as mediators — they won’t be at the bargaining table — but are going to Detroit “to help support the negotiations in any way the parties feel is constructive.” The official was not authorized to discuss private discussions and spoke anonymously.

UAW President Shawn Fain said Monday that the Biden administration won’t broker a deal.

“This is our battle. Our members are out there manning the picket lines,” Fain said on MSNBC. “This battle is not about the president, it’s not about the former president.”

Rather than launching an all-out strike of its 146,000 members, the union opted to target three factories — one at each company — a plan that could make the union’s $825 million strike fund last longer.

A key feature of the strategy is the threat of escalating the strike if the union is unhappy with the pace of bargaining. On Friday, Fain said more factories could be targeted: “It could be in a day, it could be in a week.”