Katzenbach, who served under President Lyndon B. Johnson and is best remembered for his role confronting Gov. George Wallace at the University of Alabama in 1963, said he fears long-term consequences: "If they denounce you as some kind of evil person it's going to get harder and harder to attract people into politics."
U.S. District Judge John Roll, among those slain in Tucson, had been the target of death threats after a 2009 ruling that a lawsuit by illegal immigrants against an Arizona rancher could go forward. The Arizona Republic reported the judge got more than 200 phone calls after talk-radio hosts criticized his ruling, prompting authorities to give Roll's family protection.
Some drew parallels between Saturday's events and the bombing of the Oklahoma City federal building during a period of heightened anti-government sentiment.
President Bill Clinton sounded a note of caution in a New York Times opinion piece on the 15-year anniversary of the bombing last April, writing, "We are again dealing with difficulties in a contentious, partisan time."
"As we exercise the right to advocate our views, and as we animate our supporters, we must all assume responsibility for our words and actions before they enter a vast echo chamber and reach those both serious and delirious, connected and unhinged," Clinton wrote.
Democrats, of course, have contributed, too. In 2004, the Democratic Leadership Council posted an election map, with a graphic of the country labeled "Behind enemy lines" that featured bull's-eye targets over nine states. They also posted links to stories mentioning Obama's 2008 campaign comment, "If they bring a knife to the fight, we bring a gun."
In a sign of how difficult it can be to escape the cycle of escalated rhetoric, the news out of Arizona prompted harsh attacks against Palin. It took only a few minutes for partisans on the Internet to remember that Giffords' 8th district was among those on Palin's list, and for bloggers to screengrab the rifle-scope image and spread it around. A Palin aide said in a radio interview that it was "obscene" to suggest a link between Palin and the shooting, but Internet commenters flooded Palin's Facebook page, posting vicious complaints about her that were instantly being erased.
"Mission accomplished, Sarah Palin," Daily Kos founder Markos Moulitsas tweeted less than an hour after the nation learned of the bloodshed in Arizona.
But even as people cautioned that cooler heads should prevail, the opposite happened. Instead of taking a step back to evaluate discourse in America, the tone got even hotter.
A left-leaning political activist tweeted directly at Palin that he thinks she could never run for president. He wrote, "You gotta walk into EVERY coffee shop in Iowa. With Christina Taylor Green's blood on your hands," referring to the child killed at Giffords' event.
Tea party leaders decried the shootings in Arizona and pushed back against any guilt-by-association. But after the group was "attacked" for the shootings, Judson Phillips of Tea Party Nation wrote this weekend that the era of agreeing to disagree was over.
Former Sen. Scott Brown, R-Mass., candidate for U.S. Senate in New Hampshire, holds his hand over his heart during the singing of the national anthem as he waits to take the stage for his town hall campaign rally with Sen. John McCain at the Pinkerton Academy in Derry, N.H., on Monday, Aug. 18, 2014.