I've seen progressives red-faced angry at tenure when it was in their way, so let's not kid ourselves.
The classical Japanese literature professor teaches 3 classes per year, with 8 students per class while the anime class has 300 students and a 400-student waiting list? Can't hire somebody to teach all the popular classes because of all the Milton, Spencer, Shakespeare, etc., faculty? Or when the German Dept. has a tenured Dutch specialist, medievalist, Norwegian folklore expert, and and historical dialectologist but sociology needs tenure lines? Or when tenured faculty make $125k/year while the adjuncts teaching Afro-Caribbean Literature or "Feminist approaches to literature" get $3k/course--and the senior tenured pay increases are often more than $3k/year!? And that guy saying that many of the environment problems due to population growth are because of immigrants--a thinly veiled anti-Latino racist "dog whistle"! And what to do when 3% salary increases for high-priced tenured faculty make hiring even new adjuncts impossible?
Them's the arguments in a nutshell.
Tenure means inflexible staffing.
Tenure means budgetary inflexibility.
Tenure means unpopular words can be uttered without fear of losing your job.
Tenure means that you're not being taught by people like you or whom you like, on account of politics or ethnicity or age or gender.
Given that professorships skew left on average, the "unpopular words" concern is mostly a (R) thang, but not entirely. The first two are mostly administrator issues, but that's because the left-of-center complaints are fielded on campus and seldom break out into the news. Esp. on DU, which, like most groups, defends its own.
Given Walker's past, it's hard to tell if he's out for budgetary control or political control. Often with him the two coincide: control is control. However, the political problem is that he's vying for control, which many students and administrators also want and consequently hate tenure just as much. (Yes, many students will say they like tenure, but really want to get rid of all these old professors who teach out-of-date courses. They're students and miss the fact that what they're really saying is "we like tenure when it suits us, but hate it when it's in our way."