A bipartisan Senate group has agreed on a sweeping legislative proposal that would represent the most ambitious overhaul of the U.S. immigration system in three decades. The Washington Post will be examining portions of the bill on Post Politics in a series of blog entries.

Undocumented Mexican immigrants walk through the Sonoran Desert after illegally crossing the U.S.-Mexico border. (John Moore/Getty Images) Undocumented Mexican immigrants walked through the Sonoran Desert this month after illegally crossing the U.S.-Mexico border. (John Moore/Getty Images)

The proposed immigration bill lays out a detailed, ambitious and extremely expensive plan to secure the U.S. southwestern border, will special emphasis on the porous stretch south of Tucson, Ariz. It would earmark more than $5 billion over five years to curb illegal border crossings by adding 3,500 federal border agents, building new fences, increasing aerial and ground surveillance, deploying members of the National Guard to assist in border security and speeding up the prosecution of people who are caught crossing illegally.

Yet experts have said there is no reliable way to measure whether the border is actually secure, and the vast discrepancy in statistics and measurements cited by U.S. officials and their opponents on the issue is truly startling. The Department of Homeland Security says that illegal border crossings have fallen to their lowest level in decades -- dropping to a "net" level of zero as deportations have reached a record high -- especially after the Obama administration dramatically pumped up its border security budget and personnel, from  $5.9 billion and 28,000 agents in 2004 to $11.8 billion and 41,000 agents in 2011.

On the other hand, critics such as Republican Sen. Lamar Smith (Tex.) cite other government statistics showing that only 44 percent of the border is under “operational control” of U.S. agents, and that less than 7 percent is under “full control.” The bill says that the government must achieve surveillance along the entire 2,000-mile, U.S.-Mexican border and catch 90 percent of those who try to cross certain high-traffic sectors, before any legal status can be granted to illegal immigrants already in the United States. But as long as the war of selective statistics continues, that would seem to be an empirically unprovable, and financially costly, goal.