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I have two LOR right now. One from a congressmen I worked for over a summer and another from a junior college professor who I ended up taking like 6 classes with. Problem is I never got one from a professor at Berkeley, where I ended up graduating from like 2 years ago. I wasn't thinking about going to law school back then and never cultivated any relationship with a professor (big classes, rarely went to office hours, I don't think anyone knew my name)

I checked which professors are teaching this semester and I think maybe one or two are teaching now and I don't think I'd get a very meaningful one if I just showed up and asked.

How important would that last letter of rec be?

You should probably still try and find another academic lor in some class you got an A in. Your congressman lor won't really help or hurt. Depending on schools' preferences they might want or need just 1 academic and/or 1 work. Some want 2 academic, etc.

Lors generally won't make or break your file though so don't stress, but I'd try to get another from school if I were you.

I think you should be ok as long as you already have one great lor from a professor. It would be wise to at least try and get another one though.