The Senate Tuesday defeated two amendments aimed at strengthening border security in the Gang of Eight immigration reform bill, including a provision that would have required a double-layer fence on the U.S.-Mexico border.

The back-to-back defeat of the two provisions was a blow to critics of the immigration reform measure who say it lacks meaningful border security provisions.

Only 39 senators voted for an amendment authored by Sen. John Thune, R-S.D., that would have mandated the completion of the fence along a 750-mile strip that divides the United States from Mexico before any of the nation’s estimated 11 million illegal immigrants could obtain a green card.

The Senate also voted 58-36 to kill a provision offered by Sen. David Vitter, R-La., that would have required that a biometric visa system be implemented before illegal immigrants can obtain legal status.

Thune’s provision would have required completion of at least half the length of the fence before illegal immigrants could obtain legal status.

“This would ensure that the border is more secure before any legalization program is carried out,” Sen. Charles Grassley, R-Iowa, said during Senate debate in support of Thune’s amendment.

But Democrats said the amendments would impose a lengthy delay in the legalization process and Republican members of the Gang of Eight sided with them in an effort to maintain the Democratic support needed to pass the bill.

All four GOP members of the gang — Sens. John McCain and Jeff Flake, both of Arizona, Lindsey Graham, of South Carolina, and Marco Rubio, of Florida — voted against the fence and visa amendments.

“Despite promising enforcement first, the Gang of Eight led the opposition,” Sen. Jeff Sessions, R-Ala., a leading critic of the immigration reform bill, said after the vote.

Rubio had argued for both the fence and the biometric visa system. But in a statement issued Tuesday, Rubio said he voted against the Vitter bill because it “delays the process of submitting illegal immigrants to background checks and the imposition of fines for having violated our immigration law.”

Thune’s amendment, Rubio said, “does not detail a specific border plan.”

Rubio said he is working on how own amendments to strengthen border security in the bill.

The legislation as it stands now includes $6.5 billion to boost border security, including $1.5 billion for additional fencing, but the bill leaves the construction to the discretion of Department of Homeland Security, which under the current leadership of Janet Napolitano has little enthusiasm for adding to the 36.3 miles of double-layer fencing that now exists.

Vitter argued that his amendment is an important tool for combating terrorism.

Vitter’s provision would have required every land, sea and airport entry and exit to be monitored under a biometric tracking system so that those who overstay their visas can be caught.

The commission formed to examine the Sept. 11, 2001, terrorist attacks called for such a system to be implemented.

“The 9/11 terrorists were visa overstays,” Vitter said, making the case for his amendment on the Senate floor. “We must put this in place as we act on immigration and this amendment would get that done.”

But Democrats argued that the provision would slow down the legalization process for those who are now here illegally.

“A fully biometric system is the kind of unrealistic trigger we can’t adopt,” said Sen. Patrick Leahy, D-Vt. “This amendment would be prohibitively expensive and cause all kinds of delays.”

Democrats argued that biometric visa system would cost $25 billion to fully implement, but Republicans say that estimate is too high.