MSNBC has named Alex Wagner to succeed Rachel Maddow as the weekday host of its 9 p.m. hour four days a week, turning to a political news veteran to fill one of the network’s most important time slots.

Ms. Wagner, 44, is a fixture of Washington journalism with roots in progressive news and opinion outlets. She worked for MSNBC as a host of a daytime show for several years, and rejoined the channel in February as a senior political analyst and guest-anchor during prime time, filling in for Ms. Maddow and Chris Hayes.