Democrats have a hefty advantage in national polls. President Donald Trump is playing defense, campaigning through deep-red corners of the country that he won by overwhelming margins just two years ago. And assumptions that Democrats will win control of Congress have become conventional wisdom in many quarters, from longtime political observers to social media to the oddsmakers in prediction markets.

But as Democratic dreams of midterm victories grow bigger, some strategists quietly worry that expanding expectations are crowding out signs of a more challenging political reality.

Control of the Senate hinges on victories in red states that favored Trump by significant margins in 2024, including some that Democrats have not won in decades. Republicans are outspending them by eye-popping amounts in key races.…