Some states give bipartisanship a try. It’s been a rough start.

As parties are grappling with more extreme wings, some state legislatures have responded by forming centrist coalitions across the aisle. To work, however, the model requires trust.

As state Rep. Jim Gregory walks into the Pennsylvania Capitol’s cafeteria, a table of Democrats all stops talking and turns their heads. One stands up to give him a pat on the back as he passes by.

Mr. Gregory, a Republican, ruefully remarks that this reception was likely warmer than what he’d be getting from Republicans in his home district. He’s not wrong....