The common assumption, O'Rourke told me recently, "is that the border is not secure." In fact, by almost any measure -- crime, unauthorized border crossings, resources devoted to border patrol -- the U.S.-Mexico border has never been more secure than it is now.

The problem for the immigration debate is that those who claim we need more border security are rarely called upon to prove it. No one has proposed a set of concrete standards; rather, some are calling for a subjective evaluation to be made by border-state governors, some of whom have political incentives to exaggerate the threat -- and track records of doing so.

Meanwhile, there's a downside to the increasingly militarized border, O'Rourke claims. In human terms, it results in more deaths. In fiscal terms, it wastes federal-government dollars that could be put to better use. And in economic terms, long wait times at the border -- due in part to the zealous but not very effective pursuit of contraband -- stifle the flow of trade that is a major driver of the U.S. economy.

Does the Mexican border need more security? Here are a few facts to consider.

* American border regions are not crime-ridden. El Paso and San Diego were America's two safest cities with populations over 500,000 in 2012, according to CQ Press. In 2010, at the height of Mexico's drug war, Juarez recorded more than 3,000 murders; El Paso had just five. On average, violent crime rates in U.S. border regions are lower than those of the rest of the nation.

* Terrorists are not coming over the border. There's never been a reported case of a terrorist attack in the U.S. that involved someone coming across the Mexican border. A congressional subcommittee report on the threat of cross-border terrorism cited unsubstantiated claims and three specific cases, including a Tunisian cleric caught hiding in the trunk of a car in San Diego who was not accused of involvement in any terrorist activity. The other two also were not linked to specific terrorist plots.

* There are not "bombs exploding in El Paso" or "decapitated bodies in the desert." The former claim was made by Texas Gov. Rick Perry in 2010, the latter by Arizona Gov. Jan Brewer the same year. But Perry appeared to be talking about something that happened in Mexico, not the U.S., and Brewer's claim couldn't be substantiated -- she later said she misspoke. The Senate plan proposes giving border-state governors, among other officials, the final say over whether the border is secure enough to trigger a path to citizenship -- a subjective measure that highlights the lack of a quantitative standard for security.

* The border patrol is bigger than ever. The number of agents has quintupled in the past two decades, from 4,028 in 1993 to 21,394 in 2012. In 2010, the federal government spent more than $17 billion on customs and enforcement at the border. The billions of dollars already devoted to border security pale in comparison to what it would cost to build a fence, as many politicians advocate: In Texas alone, a border fence would cost an estimated $30 billion.

* Illegal border crossings are at a 40-year low. The number of migrants caught by the Border Patrol is down 61 percent since 2005 and is at its lowest level since 1972. The average agent catches just 20 migrants per year. More Mexicans are now thought to be leaving the U.S. than entering each year, largely due to the stalled American economy. (Nonetheless, the number of illegal crossers caught each year is greater than 300,000.) Meanwhile, more migrants are dying in the desert, as stepped-up security forces would-be crossers into more and more inhospitable areas.

Borderfactcheck.org

* Lots of drugs do still come across. Narcotics seizures on the border are at all-time high levels, according to the Justice Department. "While this indicates more effectiveness at stopping drugs, it also shows that traffickers are not being deterred" by present levels of security," notes the Washington Office on Latin America, an American non-profit.