Farming, Tech, Fossil Fuels, Civil Rights, etc. Maybe as many as 50.

Each one puts up candidates, perhaps on a list basis,

You give everyone 10 votes to distribute to those candidates. If you're really into supporting Farming, you might put 10 votes to Farmers and screw the rest. If you are more widely concerned you might put a couple in tech, a couple in civil rights, one in space, etc.

Those votes are then distributed and the number of representatives are chosen in proportion. If Farming gets twice the votes as Tech, they have twice the congressmen, and Farming gets twice the representation at a national level.

If you don't have this, you end up with widely supported low level things (say a 15% support evenly across the country - truckers for example) with no representation, but areas where there are high levels of concentrated support (Tech for example) with a lot of representation.