The schedule is especially packed in New Hampshire, an early presidential nominating state where there is a competitive Senate race Democrats are flocking to this month.

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The incidents, the first of which was reported on Thursday, came as election officials have reported a growing number of threats directed at them.

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Two major Republican super PACs are said to be withdrawing cash from the races of the incumbent Representatives Thomas H. Kean Jr. of New Jersey and Mariannette Miller-Meeks of Iowa in an effort to triage the midterm map.

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