DURING THE past 14 years, under presidents Bush and Obama, the United States has sent more than 9,000 additional Border Patrol agents to the Southwest frontier, more than doubling the force there. It has built more than 600 miles of fencing; installed almost 12,000 underground sensors; and deployed scores of aircraft, drones and boats, to say nothing of an array of other technically advanced surveillance systems that enable agents to see much of what moves along the border with Mexico.

Not coincidentally, in the same 14-year period, the number of undocumented immigrants detained by Border Patrol agents — a reflection of overall illegal crossings from Mexico — has been more than cut in half, from 1.6 million in 2000 to fewer than 500,000 in the fiscal year that ended a few weeks ago, according to recently released figures.

All this appears to have been lost on Republican candidates like New Hampshire Senate hopeful Scott Brown, among others. Mr. Brown has been trying lately to claw his way into office by scaring the bejesus out of voters, portraying the border as wide open to Ebola, terrorists (read: the Islamic State) — and, oh yes, illegal immigrants, too.

Desperate to conjure up arguments against immigration reform, much of the GOP has long been blind to the buildup and militarization along the border — even though it has been achieved by bipartisan authorizations of cash by Congress. After all, the standard rationale for opposing legal status for the nation’s 11 million undocumented immigrants is to insist that the Southwest border first be “secured.”

In fact, the border has never been, and never will be, sealed, or even “secured” to the satisfaction of Republicans who use it as an excuse for doing nothing to fix the nation’s broken immigration system.

It’s true that the number of illegal border crossings spiked by about 15 percent in the year ending Oct. 1, an increase mainly attributable to a spring and summer surge in unaccompanied children. Those numbers have fallen back to historic norms, thanks to stepped-up interdiction by Mexican authorities and an intensive PR campaign warning families to stop sending their unaccompanied minors to the United States.

Meanwhile, the United States has deported more than 2 million illegal immigrants since Mr. Obama took office — more in 5½ years than President Bush deported in eight. The 438,000 deported in 2013, the last year for which records are available, was a record — one that earned Mr. Obama the enmity of pro-amnesty groups, which took to mocking him as the “deporter in chief.” Partly as a result, the number of illegal immigrants in the United States has not grown since 2006.

If Mr. Brown were truly interested in further securing the Southwest border, he would support the immigration bill passed last year by the Senate, which devotes even more resources to an already militarized frontier while providing rationality to the nation’s immigration laws. He opposes it, finding it easier to run a campaign based on fear than on facts.