A small group of U.S. national security officials has been discreetly advising state governments and congressional lawmakers on how to prepare for the possibility that the Trump administration will attempt to interfere in November's midterm elections, according to people involved in the effort.

The group includes about 10 current and former officials with experience in the Defense Department, some of whom have previously focused on countering authoritarianism abroad.

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Their efforts are motivated not by political bias, they say, but a growing sense of alarm that President Donald Tru...