While suburban congressional districts are swelling with new residents, lawmakers in large swaths of rural America and some Rust Belt cities are in need of more people to represent.

In rural Illinois, Republican Rep. Mary Miller's district is short 73,000 people. In northeastern Ohio, Democratic Rep. Tim Ryan needs an additional 88,000 people. And the Detroit-area district of Democratic Rep. Rashida Tlaib lacks over 100,000 people — one of the biggest shortfalls in the country.

That makes them all potential targets for map makers — and possibly vulnerable to job loss — as their districts are r...