In Nevada, the protesters took the stage and interrupted Senator Bernie Sanders as he addressed a crowd of supporters last month.

In Los Angeles, they disrupted Senator Elizabeth Warren’s stump speech this week.

And on Super Tuesday, activists campaigning against the dairy industry had their highest-profile moment yet when two women rushed onstage and crashed former Vice President Joseph R. Biden Jr.’s rally in Los Angeles, screaming and holding images of a wide-eyed calf.

Photos and videos of Mr. Biden’s wife, Jill, and a senior campaign adviser, Symone Sanders, pulling the women offstage circulated online and on television morning shows, leading perplexed viewers to ask, “What is ‘Let Dairy Die,’ and why is it all over the Democratic primary race?”

It is a campaign led by Direct Action Everywhere, a coalition of animal rights activists who are trying to stop an industry that they say enslaves and tortures animals at the expense of the environment. The group has for years released videos, many taken surreptitiously, trying to expose bad practices in the meat industry.

“The dairy industry is one of the cruelest forms of animal exploitation and our governments are endorsing it,” a Facebook post that describes the organization’s mission statement says. “We must call attention to this injustice.”