MSNBC’s key 9 p.m. ET primetime weeknight slot has been mired in a ratings slump since “The Rachel Maddow” show shifted to a once-per-week format, according to Nielsen data.

Since the start of the month, the left-leaning network has aired “MSNBC Prime” in the critical time slot four nights per week – with its star anchor Maddow taking the reins to host her show only on Mondays.

That shift has coincided with a downturn in MSNBC’s primetime ratings – especially among viewers in a key advertising demographic. “MSNBC Prime” has averaged about 1.2 million viewers since its debut on May 3, down from the 1.7 million viewers Maddow averaged on her show before the shift to a once-per-week broadcast.