LAS VEGAS — Hillary Clinton came here on Thursday to talk jobs, debt-free college, renewable energy and other planks of the Democratic platform.

But it was another cause, one the mainstream left generally overlooks, that intruded on her 24-minute speech to members of the International Brotherhood of Electrical Workers. Protesters chanting about animal rights pressed close to the union hall stage and briefly stopped her from speaking. Their signs read “Until Every Animal Is Free.”

As Mrs. Clinton held an impassive gaze, one burly Secret Service agent, his head shaved and wearing a purple shirt and cuff links, put an arm around the Democratic nominee and protectively patted her shoulder.

“Keep talking,” the agent said into her ear. “You handled it.” His words were captured by the lectern microphone.