I was a student at Harvard Law School (HLS) in the 1980s, and can tell you generally how it worked back then. The HLS admissions office kept statistics on all undergraduate institutions from which students had been admitted over the years, including subsequent law school grades of students admitted from each undergraduate institution. Based on those statistics, undergraduate GPAs of applicants were adjusted to account for differences due to, for example, grade inflation or academic standards at various schools. For undergraduate institutions for which HLS had very little information, LSAT scores and other factors were likely to be given greater weight (and, presumably, there would be less flexibility in admitting students whose undergraduate records deviated substantially from 4.0).
A plurality of my law school classmates were undergraduates at Harvard, and a rather high percentage were undergraduates at Harvard, Yale, or Princeton, but as I look through my first year class, I see others who graduated from Transylvania (!!), Millsaps, U.C. at San Diego, Glenville State, Lawrence, Wabash, Harpur, U.C. at Santa Cruz, Presbyterian, Augustana, Southern Illinois, La Roche, South Carolina College, and U. of Texas at San Antonio. I had at least two or three classmates from U. Wisc. at Madison, but I doubt that coming from the Madison campus gave them any huge advantage in the admissions process. (In fact, I remember one of them telling me that, if he had gotten a single "B" during his years at Madison, he wouldn't have been admitted to HLS.)
Regardless of whether you're at the Madison campus or elsewhere, you're probably going to need an undergraduate GPA in the range of about 3.9-4.0. I would expect your LSAT score to be somewhat more important if you're at one of the smaller campuses.
Personally, I would think that transferring to another campus at this point would do more harm than good -- at least, insofar as law school admissions is concerned. If you're a strong candidate for HLS, then you should be developing a good reputation among the faculty and others at your current campus. Transferring to Madison might just make you into another unknown undergraduate at a big campus. If you're happy at your current school, and you're getting a good education, then I don't see much upside to transferring.