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The onus falls on South Africa, Mr. Mugabe’s southern neighbor, with which he has cordial ties.

“South Africa needs to step up to the plate,” said the congressional source.

Earlier this month, Mr. Mugabe dismissed South African President Jacob Zuma’s international relations adviser, Lindiwe Zulu, as “a stupid, idiotic street woman” — a local reference to a prostitute — after she questioned Zimbabwe’s election preparedness.

Mr. Zuma later rebuked his aides for making “unfortunate statements.”