President Trump and Senate Democrats are steadfastly opposed to one another over judicial nominees. Sens. Debbie Stabenow, D-Mich., and Gary Peters, D-Mich., are now threatening to “blue-slip” Trump’s nomination of Joan Larsen to the 6th Circuit Court of Appeals. Other senators are making similar threats. With Republicans poised to reform the blue slip, now seems an appropriate time to discuss what it is and how it works in practice.