Hillary Clinton was rushed at by an animal rights protester during a speech on the economy, the second time in a week.
Ms Clinton was talking in Des Moines, Iowa, and facing the other way as a woman launched herself over the barrier and towards the stage.
She has been named as 26-year-old Kelsey Atkinson from campaign group Direct Action Everywhere, which fights for "total animal liberation".
Just as the protester got to the steps, Secret Service staff pulled Ms Atkinson away.
In a Facebook post, the group said Ms Atkinson and her colleagues wanted to challenge Ms Clinton to "take a stand against animal torture and stop taking money from animal-exploring corporations like Costco".
Ms Clinton recently held a fundraising event with Costco founder Jeff Brotman. Tickets for the event cost $50,000.
Ms Atkinson said in a statement: "Hillary is taking a huge amount of money from companies like Costco, and when government money goes back to their suppliers, it raises serious questions about undue influence."
"If we’re serious about animal protection, we need to stop funding animal torture and start the process of abolishing the systems that exploit them."
All candidates have been targeted by protesters, including Donald Trump who was interrupted at least 15 times during his key economic speech in Detroit this week.
This is the second time the Democratic nominee has been targeted by animal rights protesters after a similar incident last week in Nevada.
Ms Clinton has also come under fire with Greenpeace activists, who challenged her at rallies and prompted her to say she was "sick and tired" of Bernie Sanders' "lies" about her climate change policies and donations from fracking and oil tycoons.
