Saturday’s attack, which left seven people dead, marked the third major terrorist strike in Britain in as many months — the first unfolding steps from Parliament and the second outside a packed pop concert in Manchester. Each was claimed by the Islamic State.

The latest assault, in which three suspects mowed down pedestrians on London Bridge before slashing their way through a nearby market, inserts an unpredictable new dynamic — the fear and uncertainty sowed by terrorism — into this week’s contest, which was already tightening.

Once projected to end in a landslide for Theresa May, the Conservative prime minister who called the election in a bid to consolidate her majority, the race has appeared less lopsided in recent days. Polls suggest it could even offer a lifeline to Jeremy Corbyn, the firebrand Labour chief whose leadership had been in doubt as his party struggled to gain traction.

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