The Montana case is linked
here. Whether that ruling qualifies as a direct challenge to
Citizens United depends on who you believe. The majority in the Montana case determined that
Citizens United wasn't controlling based on what they considered significant differences between the laws under consideration in the two cases. The dissenters in the Montana case said that
Citizens United is a shitty decision but controlling nonetheless.
The Montana decision just came down, and the losers haven't yet sought SCOTUS review. When they do, I suspect that they'll have a much better chance of getting in than most.