WINNSBORO, Tex. — Deep within the oak and pine forests that blanket this stretch of East Texas, the chug of machinery drones on late into the day, broken only by the sounds of a band of activists who have vowed to stop it.

Here, among the woods and farmland, what might be one of the last pitched battles over the Keystone XL oil pipeline has been unfolding for weeks now, since construction of the controversial project’s southern leg began in August.

As bulldozers and diggers churn up a 50-foot-wide path for the pipeline — this portion will run from Cushing, Okla., to the Gulf Coast — a small group of environmental activists have taken to the towering trees in its way.

And with the blessing of some landowners who live here, and whose property the pipeline will cross, the protesters have fashioned a web of tree houses, structures and pulleys in a last-ditch effort to keep the enormous project from rumbling forward.