President Donald Trump is heading to Kansas on Monday and Iowa later in the week, the latest in a series of stops where he's trying to energize his base in the final weeks before the midterm elections. The economic strain is becoming a growing political concern for Republicans in states they have long dominated.

Federal authorities are investigating letters that have been mailed to congressional and election offices nationwide and are urging recipients to treat the mailings as "potentially hazardous."

The shifting money in the final weeks of midterm campaigns points to a defensive posture by Republicans and new aggression from Democrats, who aim to capitalize on President Donald Trump's weak approval ratings.