With today's shooting of Republican lawmakers, staffers, and law enforcement, commentary has lapsed into the black hole created by the Fundamental Problem of Terrorism (Studies). That is, there exists no agreed-upon definition of terrorism in government or academia. The result is that the determination of acts as terrorism is highly interpretive. Hence, the tendency of media to call only violence perpetrated by Muslims as terrorism and violence by whites typically as hate crimes. Or why much ink and many bytes are spent on determinations of the sanity of would-be terrorists in order to properly classify events.
The intensity of the debate for each event suggests the political nature of labeling terrorism. Of course, from a legal perspective it matters--terrorism charges have specific evidentiary standards and legal consequences. From an academic perspective, it may not matter that much.
Terrorism or not, the use of violence towards political ends is dangerous beyond the act itself. First is the degradation of norms that political processes are not debated with violence (a tenuous norm if one considers the many, many ways that violence is inflicted via politics), that could lead to a threshold whereby other would-be users of violence would turn to action. The works of Schelling and Granovetter still dominate here.
The second pernicious effect lays in the utility of terrorist violence itself. As David Lake offered, terrorism is used to shift the bargaining range to something more extreme than before the violence. Further violence of this nature pulls discourse away from centrist positions. As it appears that today's attack came from the left side of the political spectrum and with existing and continuing right-wing violence, political bargains will only be found where the extreme tails of each side overlap, making them less and less likely.
This surely is patently obvious to scholars of political violence. But the point is that the labeling is only marginally important compared to the acts themselves. There's no point in arguing in circles about this being terrorism or not while our politics spin into a more-violent orbit.