The amendment, which co-sponsor Sen. Lindsey Graham, R-S.C., said would "almost militarize the border," would reduce the number of people entering the United States from Mexico without legal permission by one-third to one-half, "compared with the projected net inflow under current law," the Congressional Budget Office said.
The original version of the Senate "Gang of Eight" bill would have reduced illegal immigration 25 percent.
The amended, "border surge" version of the bill would double the U.S. Border Patrol force to 40,000 agents and complete 700 miles of new border fencing, up from 350 miles lawmakers originally considered.
The additional border guards and fencing, plus new high-tech border surveillance, would cost about $38 billion, said the CBO, whose report can be found at tinyurl.com/UPI-CBO-Immigration.
The drop in illegal immigration would not come right away, the CBO cautioned, "as it would take several years before DHS [the Department of Homeland Security] could hire the full number of Border Patrol agents called for in the act."
But even with the increased spending, the bill that passed the Senate 68-32 a week ago would cut the federal deficit $158 billion between 2014 and 2023, the CBO said.
The original bill was forecast to save $197 billion.
From 2024 to 2033, the Senate-passed bill would cut the deficit $685 billion, the CBO said, down from the pre-amended bill's estimated decrease of $700 billion.
"CBO has reaffirmed that immigration reform reduces the debt and grows the economy," he said, adding the CBO methodology in determining the future illegal-immigration rate "underestimates" the actual decrease.
Critics of the Senate bill -- which includes a pathway to citizenship for 11 million immigrants in the United States illegally -- said Wednesday the CBO's calculations proved the legislation doesn't go far enough.
"Now CBO says that the Senate bill, with all its promises about extra enforcement, would still allow half the illegal immigration flow to continue," NumbersUSA Executive Director Roy Beck said in a statement.
"This CBO finding suggests that by 2020 the illegal population will have grown big enough that presidential candidates will once again be pressed to promise yet another amnesty to 'fix a broken immigration system,'" he said.
House Republicans are due to meet Wednesday to discuss how to proceed on immigration policy. They have said they will not take up the Senate measure but instead will focus on narrower legislation that would not provide a path to citizenship for the immigrants living in the country illegally.