While there is a pattern in the left's repeated imputation of political motives to and shameless political exploitation of many large-scale violent acts in this country, there is a larger pattern at work, as well. That is, the left doesn't merely attribute violent acts to conservatives; increasingly, it frames their stance on most policies as their being morally depraved, which is why such statements as "Republicans want a smaller America" flow so freely off the tongue of President Obama.
According to this view, which is shockingly more pervasive on the left than the uninitiated might suspect, conservatives want to protect the unborn because they disrespect women and women's rights; they oppose same-sex marriage out of bigotry; they support welfare reform not to help people in the long run but because they lack compassion; they oppose higher tax rates for all income brackets because they believe in carving out special privileges for the wealthy; they advocate reducing the federal government's role in education because they don't care about the poor or minorities; they support stricter immigration law enforcement because they are bigots and nativists; they pass voter ID laws because they are racists who want to suppress the minority vote; they oppose socialized medicine because they don't care about those with pre-existing conditions or those without health insurance; they favor enhanced interrogation techniques against terrorists not to save innocent lives but because they are bereft, to some disturbing degree, of humanity; they obsess against the United Nations because they are selfishly nationalistic and close-minded; they oppose limitless extensions of unemployment benefits not because such extensions have been found to exacerbate unemployment but because they don't care about people; they favor the death penalty, gun ownership rights and a strong national defense because they are violent, jingoistic, imperialistic warmongering cowboys; and they love corporations and even want dirty air and dirty water -- another actual Obama quote -- because profit is their god and people come in a distant second.
Do you see the irony? The left's entire premise about Oklahoma City, Tucson and Aurora is that the right's rhetoric is hateful and leads to division and even violence because it gets people thinking such terrible thoughts about others. Yet on so many policy disputes, the left demonizes conservatives in precisely those terms.