While Carrol is right to think if we could install PR (or other electoral reforms ) at the national level, we would have the power to do a great many things . But I think we could win on this type of issue at the STATE level. Leftists in the U.S tend to forget how much power the States have. In theory, at least, we could install PR for election of State legislatures and local bodies, plus instant run-off for offices such as Mayor and Governor which are inherently yield a single result. By the same theory we could win progressive taxation, single payer health , anti-racist measures, abortion and gay rights to a large extent at the state level.
How would this work in practice? Don't know -- the reactionaries have certainly gained many of their victories at the local and state levels, via initiatives, electing schoolboard, electing state legislatures. Of course they have the reactionary churches plus virtually unlimited money.
I know we can all think of examples where leftists have tried this and failed (for example California's single payer health initiative). On the other hand I think we could argue that at least part of the failure of the California single payer initiative was due to the top-down, advertising driven means that were used to push it. A truly grassroots based effort, one that used people to people contact to bypass the media might have a shot. If something like single payer did win at a state level then the left would have truly delivered something of immediate benefit to just about every working person in that state -- which would be a great basis for future organization.
Where I'm putting my energy at the moment.... Seems worth a shot...