Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President George W. Bush from 2001 to 2002, he is the author of six books, including "Comeback: Conservatism That Can Win Again," and is the editor of FrumForum.
Washington (CNN) -- When people call the shootings in Tucson senseless, they are absolutely right. By all reports, the alleged Tucson shooter was seriously mentally ill and quite probably schizophrenic.
The congresswoman he shot and the judge he killed had been targets of violent threats from political extremists. But the shooter himself was not a political extremist in any meaningful sense of that term.
In this, the Tucson shooter follows a familiar pattern in recent American life.
-- A sniper terrorized the Washington, D.C., suburbs in the fall of 2002, killing at least 10 people. It was widely assumed that the motive was Islamic terrorism, an assumption strengthened when the killer was identified as a convert to the Nation of Islam. Yet while John Allen Muhammad used the language of jihad, he seemed at least as motivated by revenge against his ex-wife and greed for the ransom he hoped to extort from the U.S. government.
-- In October 1994, gunman Francisco Duran fired 29 shots over the White House fence at a group he believed to include President Clinton. Duran later explained that he was trying to protect the world from space aliens.
-- John Hinckley shot and almost killed President Reagan in hopes of gaining the attention of the actress Jodie Foster, with whom Hinckley was obsessed.
-- Of the two attempts on the life of President Ford, one was carried out by a member of the Charles Manson cult; the other by a supporter of the bizarre Symbionese Liberation Army.
-- The gunman who crippled Alabama Gov. George Wallace during his 1972 presidential campaign wrote a diary detailing his motive: a quest for personal fame.
The United States has a long history of more strictly political violence, from the assassination of President Lincoln through the 1960s murders of Medgar Evers, Martin Luther King and John and Robert Kennedy. And of course we have a new exposure to international terrorism, especially from Islamic extremists.
Yet since 1970 -- and despite horrific throwbacks such as the 1995 Oklahoma City bombing -- the political aspect of home-grown violence has receded.
I'd suggest three reasons for the recession of political violence.
1. Better police work
Lee Harvey Oswald actually attempted another political murder in Dallas six months before he killed President Kennedy. Because the police never solved the first crime, Oswald remained at liberty to commit the second.
A modern-day would-be Oswald faces much more sophisticated and capable law enforcement agencies, and not only at the federal level. The New York City Police Department, for example, now employs former CIA spymaster David Cohen as deputy commissioner for intelligence. Atlanta Police Chief George Turner formerly served on a Secret Service presidential protection detail.
The U.S. Secret Service receives and investigates 30 threats a day against President Obama, according to a recent book by Ronald Kessler. That figure might not be exactly reliable, but it gives an idea of the scope of modern security work.
A modern radical bent on political violence had better stay off Facebook, refrain from blogging, eschew e-mail and texting and pay cash for everything. He or she must be very careful about choosing associates.
Otherwise, he or she will make it too easy for police to penetrate the radical group and foil the crime, as regularly happens to Islamic radical terror cells operating inside the United States.
2. Racial progress
From Abraham Lincoln to Martin Luther King, the central killing ground of American politics was the battle to maintain or overthrow white supremacy over black Americans. That issue has now been decisively settled. The Black Panthers have been rendered as obsolete as the Ku Klux Klan.
Not all is sweetness and light, obviously. Ethnic competition -- even racial animosity -- remain real and often deadly in an increasingly multiethnic America. Cities such as Oakland, California, have been violently torn by violence between black and Latino gangs. But violence over the specifically political question -- "shall blacks be accorded equality with whites?" -- has faded as the question has faded.
3. The strength of the American political consensus
Last week, a bodyguard assassinated Salman Taseer, governor of Pakistan's Punjab province. That crime successfully achieved political ends. Taseer had forcefully criticized Pakistan's blasphemy laws. In reaction to the killing, Pakistan's weak government has indicated that it will back away from changes to the blasphemy laws.
The outpouring of support for the assassin from religious leaders and ordinary citizens has reoriented the government in a much more explicitly Islamic direction. Coalition partners have deserted the governing party. The political killing "worked." Pakistan can expect more.
By contrast, political killing in the United States never works. James Earl Ray unwittingly contributed to the elevation of Dr. Martin Luther King Jr. to a national symbol and confirmed civil rights as the law of the land.
The violent left-wing extremists of the 1960s helped elect Richard Nixon in 1968 and re-elect him in 1972. Violence against abortion clinics discredits the pro-life movement and ACT-UP's desecration of Catholic churches threw back the cause of gay rights.
Sane Americans appreciate this basic fact, which is why sane people -- even when quite politically extreme themselves -- eschew violence. Who wants to end up like Bill Ayers?
But this fact carries a moral. Gov. Taseer's assassin committed murder because he (correctly) believed he would find allies. American would-be assassins are deterred because they know they will meet only execration and exclusion.
Would-be assassins must continue to know that. Which is why it is so dangerous when influential voices in the public sphere say things that crack the anti-violence taboo.
We can number the milestones:
-- The novel published in 2004 (sadly, by the respected house of Knopf) in which characters debated the assassination of George W. Bush.
-- The paranoid "truther" mania that accused the U.S. government of self-inflicting the 9/11 attacks.
But the election of Barack Obama and the pain of the economic crisis have opened the door to worse, if worse was possible.
I wrote a column in 2009 noting the spread of extremist talk:
"A man bearing a sidearm appears outside President Obama's Aug. 11 town hall meeting in Portsmouth, N.H., under a sign proclaiming, 'It is time to water the tree of liberty.'
"That phrase of course references a famous statement of Thomas Jefferson's, from a 1787 letter: 'The tree of liberty must be refreshed from time to time with the blood of patriots & tyrants.'
"Earlier that same day, another man is arrested inside the school building in which the president will speak. Police found a loaded handgun in his parked car.
"At an event held by Rep. Gabrielle Giffords in Arizona last week, police were called after one attendee dropped a gun.
"Nobody has been hurt so far. We can all hope that nobody will be. But firearms and politics never mix well. They mix especially badly with a third ingredient: the increasingly angry tone of incitement being heard from right-of-center broadcasters.
"The Nazi comparisons from Rush Limbaugh; broadcaster Mark Levin asserting that President Obama 'literally at war with the American people'; former vice presidential candidate Sarah Palin claiming that the president was planning 'death panels' to extirpate the aged and disabled; the charges that the president is a fascist, a socialist, a Marxist, an illegitimate Kenyan fraud, that he 'harbors a deep resentment of America', that he feels a 'deep-seated hatred of white people,' that his government is preparing concentration camps, that it is operating snitch lines, that it is 'planning to wipe away American liberties': All this hysterical and provocative talk invites, incites, and prepares a prefabricated justification for violence."
Deranged people do things for deranged reasons. The only protection from them -- and for them -- is an improved system of diagnosis and treatment for the mentally ill.
It's the radical but nonderanged who demand a political response. In recent years, they have been hearing the wrong thing.
This terrible crime in Tucson summons us all to reaffirm the norms of our system -- and to reaffirm that as intensely as we contest our political opponents, we respect our opponents' legitimacy and we honor their humanity.
The opinions expressed in this commentary are solely those of David Frum.