Born into rural poverty, he climbed to the top of the art market. But he fell after being convicted of selling fake and stolen items.

Mr. Pappas, a 44-year-old Democratic congressman, is the first major candidate to enter the race to succeed the retiring Senator Jeanne Shaheen.

Trump’s tariffs erect a wall between Americans and other people, obstructing the flow not only of goods but also of ideas, contacts, technology and friendships.