One sweltering morning in Phoenix, four workers from a Latino nonprofit stood with clipboards outside a motor-vehicle office to do the grunt work of democracy: persuading reluctant Americans to register to vote.

While most people took a voter application or moved on, Vlad Stepanov, a conservative video blogger, stopped when he saw the canvassers. To Mr. Stepanov, 33, the sight of them in matching “Poder Latinx” T-shirts, meaning Latinx Power, in a heavily Latino neighborhood of Phoenix was suspicious.

As his mother quietly filmed, he strode up to the canvassers. Were they registering noncitizens to vote? Who was funding them?

The canvassers insisted they were following Arizona laws and registering only citizens. “When you get your ID, we can help you register,” one told him.

“But that doesn’t prove I’m a citizen,” Mr. Stepanov responded.

It was another contentious day in the trenches of a bitterly fought election. As the ground game intensifies ahead of many state voter-registration deadlines in early October, suspicions of election fraud have turned the normally ho-hum work of registering voters into tense confrontations.

Despite the many debunked falsehoods about widespread voting by noncitizens, liberal Latino advocacy groups say they are being trailed by conservative activists with cameras and accused of registering undocumented immigrants.