Correction: An earlier version incorrectly said that 44 percent of the Southwestern border with Mexico is not under “operational control.” According to the U.S. Border Patrol, 44 percent of the border is under “operational control.” The version below has been corrected.

DESPITE A BLOOD-SOAKED drug war that has convulsed Mexico, a broad array of data shows that America’s southwestern border is increasingly safe, secure and, thanks to measures launched by President George W. Bush and sustained by President Obama, much harder for illegal immigrants to cross. Yet against substantial and mounting evidence, Republicans in Congress continue to portray the border as beset by rising violence, out of control and a grave threat to national security.

Given the clear data, it is hard to view these scare tactics as anything but a cynical effort to distort the debate on immigration reform. The intent is to distract Americans from the problem of 11 million immigrants here illegally by pointing to an imaginary wave of crime and instability at the border. Of course, goes the argument, we need immigration reform, but we can’t possibly achieve it until order is restored.

The Republican strategy is dishonest — and effective. “This is a national security threat,” Sen. John Cornyn, a Texas Republican, told a congressional hearing. “So we need to regain the confidence of the American people before they’re going to allow us to move forward . . . to fix our broken immigration system.” Rep. Michele Bachmann, a Republican presidential hopeful, agreed with a questioner at a town forum this week that U.S. troops should be redeployed from South Korea to south Texas — a move that might comfort the North Koreans but would have little or no effect in Texas.

In fact, it is Mr. Cornyn himself, along with others who make sensational statements, who undermine the confidence about the border he says “we need to regain.” They do so with misrepresentations and twisted statistics whose effect is to obscure the dramatic drops in illegal border crossings and violent crime along the border.

One of the GOP’s favorite bits of rhetorical ammunition comes from the Government Accountability Office, which said in a report this year that 44 percent of the Mexican border is under the Border Patrol’s “operational control.” In fact, most of that 44 percent includes the border’s most remote, inaccessible, sleepy and least-crossed terrain — hardly the peril Mr. Cornyn and others make it out to be. Still, many Republicans trot out the 44 percent at every opportunity, conjuring the image of an unpoliced free-for-all on the border.

The truth is very different. Nearly 18,000 Border Patrol agents are now deployed at the border, a force that has nearly doubled since 2004, in addition to thousands of personnel from other federal agencies as well as hundreds of National Guardsmen. Thanks to that presence, as well as to economic, demographic and other factors in Mexico and the United States, apprehensions of illegal border crossers by the U.S. Border Patrol — a fair measure of the border’s porosity — have been cut by three-quarters over the past decade.

On current trends, including a 30 percent drop in the past 10 months compared to the same period of 2010, the number of apprehensions in fiscal year 2011 will be the lowest in 40 years. In other words, illegal immigration has fallen to levels last seen in the Nixon administration.

None of this is an argument for standing down or lowering the nation’s guard at the border. Like many international boundaries, the U.S.-Mexico frontier has never been impenetrable or immune to smuggling, corruption and criminality.

Today, Mexican drug cartels are waging a vicious war in Mexico that has spawned an orgy of murders in Ciudad Juarez, just across the Texas border from El Paso. Mexico’s police force is ineffective and corrupt. In some cases, drug smugglers and others attempting to enter the United States have confronted U.S. law enforcement personnel, and for years there have been instances of rock throwing, occasional Molotov cocktails and even some shootings from the Mexican side.

But Border Patrol statistics show no significant spike in violence against officers in recent years, and this year the number of such incidents is sharply down. A spokesman for the Drug Enforcement Administration told us flatly: “The Mexican drug war has not spilled over into the U.S.”

Overall crime is decreasing in most U.S. border cities, according to the latest FBI figures. Kidnappings, many of them related to drug trafficking, are also falling fast after having briefly spiked in Phoenix a couple of years ago.

Even in El Paso, overall violent crime — not very high compared to the rates in other large American cities — has not increased, and the murder rate over the past decade has fallen sharply compared with the 1990s. An uptick in murders early this year was unrelated to instability at the border, according to El Paso police.

According to the FBI, and to most coolheaded law enforcement officers in the Southwest, the violence in Mexico has not affected cities on the American side of the border. “Unfortunately,” El Paso police chief Greg Allen told USA Today last month, “some people’s misperceptions have become their reality.”

Members of Congress, such as Mr. Cornyn, have tried to have it both ways. On the one hand, as a senator with a large constituency of Hispanic voters, he’s acknowledged the pressing need for immigration reform. On the other hand, Mr. Cornyn has voted against virtually all serious legislation aimed at fixing the nation’s immigration system, including bills sought by the Bush administration.

In a congressional hearing, he pressed Homeland Security Secretary Janet Napolitano to specify how much additional “time and efforts” are needed to secure the border. But at the same time Mr. Cornyn has never defined what a secure border would look like, and he’s only glancingly acknowledged that illegal crossings have plummeted.

Horror stories about an out-of-control border are untethered from the facts. They’re also irresponsible. By using the myth of escalating border insecurity as an excuse for inaction on the pressing reality of a broken immigration system, politicians perpetuate both a lie and the national disgrace of a dysfunctional policy.