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.”
The group encourages “rescues” of animals in captivity and public protests, like the one on Tuesday. Activists have chained themselves together in front of Whole Foods Market grocery stores and staged “die-ins” at Chipotle restaurants, stormed food courts around the world, and chanted in the streets to get attention for their cause.
But the race for the Democratic presidential nomination has given the activists an especially public platform, and many of them have risked arrest and vitriol to broadcast their message.
“Last night was the perfect place and the perfect time to get eyes on a really troubling issue that is almost entirely overlooked in the political discourse of the nation,” said Matt Johnson, a spokesman for Direct Action Everywhere.
It was the sixth political campaign event in three weeks the group disrupted, Mr. Johnson said. The activists have focused on Mr. Sanders, Mr. Biden and Ms. Warren because of their past support for the dairy industry, he said.
In Carson City, Nev., Mr. Sanders was forced to step aside when a woman grabbed a microphone from him. He yanked it back, but she grabbed a different microphone.
Mr. Sanders walked away from the lectern as she said, “Bernie, I’m your biggest supporter and I’m here to ask you to stop propping up the dairy industry.”
Three other women, one of them topless, walked up to the lectern holding photographs of animals, and then were shooed offstage. Mr. Johnson said that three women who appeared without a shirt and bra at the event were arrested and charged with indecent exposure; they were each freed on $2,500 bail.
In Los Angeles, Ms. Warren appeared more accommodating when two women rushed onstage to hold up signs. She smiled at them, perhaps a little tightly, as they yelled from the stage.
The two protesters who stormed the stage at Mr. Biden’s rally Tuesday were identified as Sarah Segal 48, and Ashley Froud, 21, by Direct Action Everywhere in a statement.
“We’re asking him to stand with animal rights advocates, environmental activists, and ordinary citizens — and against the inherent violence of the dairy industry,” Ms. Segal said in the statement. Mr. Johnson said she and Ms. Froud were let go after being removed from the stage.
And the issue has become more prominent with the backing of the actor Joaquin Phoenix, who provoked the ire of dairy farmers last month when he railed against the industry during the Academy Awards.
“We go into the natural world and plunder it for its resources,” he said as he accepted the Oscar for best actor. “We feel entitled to artificially inseminate a cow and when she gives birth, we steal her baby even though her cries of anguish are unmistakable. And then we take her milk that was intended for her calf and we put it in our coffee and our cereal.”