WASHINGTON — Darkness is enveloping American politics.

With four weeks to go before the midterm elections, Republicans have made questions of how safe we are — from disease, terrorism or something unspoken and perhaps more ominous — central in their attacks against Democrats. Their message is decidedly grim: President Obama and the Democratic Party run a government that is so fundamentally broken it cannot offer its people the most basic protection from harm.

Hear it on cable television and talk radio, where pundits and politicians play scientists speculating on whether Ebola will mutate into an airborne virus that kills millions. See it in the black-hooded, machine-gun-brandishing Islamic extremists appearing in campaign ads. Read about it in the unnerving accounts of the Secret Service leaving Mr. Obama and his family exposed.

Republicans believe they have found the sentiment that will tie congressional races together with a single national theme.