The Senator's glasses are a bold look, and she wears them often. Here's another angle, from January in anticipation of the State of the Union address:
The question, of course, is whether she wears the glasses or perhaps they wear her. There are some image consultants who will suggest that dark, bold frames like this obscure the face, no matter who you are. I'm not one of them (full disclosure: my own Michael Kors frames are rather similar to Senator McCaskill's). I do think, though, that for her this color is perhaps in too strong a contrast with her coloring overall. Similar frames in a warm brown might have softened her look in a way that would work better with her hair color especially.
But frankly, I applaud her for regularly wearing her glasses at all. It's amazing to me how frequently we as women still succumb to the idea that glasses are somehow "unbecoming."