With seven weeks until Election Day, control of the US Senate is up for grabs as Democrats go on offense in red territory and Republicans look to make headway in a small number of blue-held seats. Democrats have long believed that the economic and political environment and historical headwinds against the president’s party have put a blue Senate majority — and a consequential check on the Trump administration — within sight. But entering the final campaign stretch, recent polling suggests they may need to play more defense than expected as a key matchup in New Hampshire raises new concerns. Still,...