COLCHESTER, England — Carla Hales, a Conservative candidate in Thursday’s local elections in Britain, was out last week doing campaign work and says she never sensed the man creeping up behind her until she heard a shout and took three quick punches to her ribs.

Ms. Hales, who felt such a sharp pain when she took a breath that she feared, briefly, that she might have been stabbed, never got a clear look at her assailant. But from the insult he hurled, she knew his motive.

Politics.

It is no secret that Britain’s chaotic and stalled efforts to leave the European Union have poisoned the national political debate, polarizing opinion and causing further damage to the already-tarnished reputation of the country’s governing class.

But the toxin now seems to be seeping down to local elections, where about 8,400 seats are being contested. While Thursday’s vote will have nothing to do with Brexit, the atmosphere has made campaigning an increasingly perilous undertaking for Ms. Hales and at least two other candidates for local councils who have been assaulted recently.