The author starts with some comments on politicians' use of personal attacks. While many editorialists criticize such attacks, most voters don't pay attention to them, because "everyone knows" these attacks won't continue after elections occur, and because politicians "can be excused" for this kind of "mudslinging."

The author then shifts focus: although politicians can thus be "excused," political commentators, she insists, cannot be. She supports this claim with another claim about what political commentators should be doing: engaging in "sustained and serious debate" related to politics. She points out that personal attacks don't allow this kind of debate to continue.

The argument starts by discussing politicians, but we know those two sentences don't contain the main point because the argument shifts focus immediately afterwards. The politicians are mentioned by way of contrast to illustrate a point about political commentators. You could think of the whole section about the politicians as a concession leading up to the main point: "Although politicians can be excused for mudslinging, political commentators cannot be."

We know that sentence — "Political commentators, however, cannot be" — is the main point, and not one of the sentences following, because both those sentences support that initial claim. They explain why political commentators cannot be excused for personal attacks: because political commentators should be engaged in serious debate, and because personal attacks cut off that kind of debate.