Senate Democrats have signaled they will block a Republican police reform bill that critics say offers a “woefully inadequate response” to racial disparities in American policing practices. The Senate minority leader, Chuck Schumer, and fellow Democratic senators Cory Booker and Kamala Harris, sent a letter to the majority leader, Mitch McConnell, on Tuesday urging him not to hold a key procedural vote this week on the Republican senator Tim Scott’s police reform bill.“This is a serious challenge requiring serious solutions,” the three senators’ letter says.