AFTER HAVING quadrupled in the 1990s, and more than tripled in the 2000s, the budget of the U.S. Border Patrol is apparently regarded by Republicans in Congress as a pathetic shell in need of a vast infusion of dollars. To buy GOP votes for immigration reform, Democrats have acceded to dumping billions of dollars to fortify the southwest border, which is already more secure than it has been in decades.

The amendment, offered by Republican Sens. Robert Corker of Tennessee and John Hoeven of North Dakota, is a case study in how budgetary decisions can be driven by overblown political rhetoric, not national interests.

The amendment would spend $38 billion — in addition to the $8 billion that was embedded in the original Senate proposal — on personnel and technology along the border. That is the political price that a handful of Senate Republicans have extracted to propel the bill — including legalization and a path to citizenship for 11 million undocumented residents — out of the upper chamber with enough momentum to give it a fighting chance in the reform-averse House. In this case, the ends do justify the means — even if the means are wildly profligate.

About 18,500 agents patrol the Southwestern border, up from 8,600 in 2000. That massive buildup has left some sectors of the border overstaffed. Nonetheless, the Corker-Hoeven amendment would add 20,000 more agents. With demographic, economic and social trends already dampening the supply of illegal immigrants, what in the world would all those agents do to fill their time?

The border is nearly 2,000 miles long and, in many areas, all but uncrossable because of deep river canyons and pitiless terrain. It can never be hermetically sealed, as some Republicans would have people believe. Nonetheless, the border is less porous today than at any time since the 1970s, thanks to a massive infusion of resources and technology begun under the Bush administration and continued by the Obama administration.

Last year, the beefed-up Border Patrol arrested just 365,000 illegal crossers, down from nearly 1.7 million in 2000. The economic downturn explains some of the diminished activity but not all; in fact, illegal crossings (as measured by apprehensions) had been cut in half in the eight years before the crash of 2008. Domestic factors in Mexico, including a decline in birth rates and better educational and economic opportunities, have contributed.

The refreshingly candid Mr. Corker admitted that his amendment is “almost overkill.” Massive overkill is more like it; in adding drones, hundreds of miles of fencing and high-tech gimmickry, it will turn the border into a frontier akin to the DMZ separating the two Koreas.

Is it worth it? As a means of ending the nation’s irrational, self-defeating marginalization of 11 million undocumented immigrants, the answer is yes.

But make no mistake: If the Senate bill becomes law, the profligate spending it would mandate is, in the end, the cost the nation would pay for years of alarmist and xenophobic Republican rhetoric about border security.