The U.S. Senate on Thursday rejected a Republican amendment demanding "effective control" of the southern border before allowing the immigration status of the undocumented population to be legalized.

Senators voted 57-43 against the amendment offered by Iowa's Charles Grassley, a measure critics said could have torpedoed the fragile agreement that produced the bipartisan immigration bill the chamber is now debating.

It was the only vote of the day, despite the fact that five amendments were in line for debate, two of them proposed by Republicans John Thune of South Dakota and David Vitter of Louisiana and also dealing with border security.

Senate Majority Leader Harry Reid of Nevada said that the senators will continue voting on amendments next Monday and stressed his determination to conclude the debate prior to the July 4 recess.

Reid read a letter from Alfredo Castañeda, an undocumented student in Las Vegas who arrived in the United States when he was just a baby and who, along with 800,000 other young people, has benefitted from the Deferred Action for Childhood Arrivals program, which last year temporarily suspended the deportations of qualified undocumented youths.

Reid said that that measure, however, is not a "permanent solution" and so Congress must quickly approve the reform package.

Grassley's amendment would have barred beginning the process of legalizing the estimated 11 million undocumented immigrants until the U.S. Department of Homeland Security "has maintained effective control" of the entire U.S.-Mexico border for a period of six months.

Before the defeat of his amendment, Grassley accused the Democrats of failing to deliver the promised "open debate" on amendments to the proposal crafted by the bipartisan "Gang of Eight."

Sen. John McCain (R-Ariz.), a member of the Gang of Eight, believes that border security will be able to be strengthened with this reform, but the majority of his Republican colleagues are not of the same opinion. EFE