The debate is expected to take days and will consider whether to create a foreign guest worker program, how best to seal the lengthy southern border with Mexico, and what to do with the estimated 11.5 million undocumented workers in the United States.
“The level of illegal immigration is unacceptable,” said Democrat Ken Salazar from the western state of Colorado. “Our immigration laws are not working, we have broken borders and we must fix the problem.”
Senator Salazar emphasised that after the September 11, 2001 attacks it was “critical that we get control of our borders” as a “matter of national security”, a view widely repeated by his colleagues in the past months.
Immigration reform plans has especially split President George W Bush’s majority Republican Party, which controls the House of Representatives and the Senate.
Mr Bush presented a reform plan that increases the number of Border Patrol agents and allows some undocumented workers to register for legal status, and ultimately, if they qualify, for US citizenship.
But hard-line Republicans in the House of Representatives in December approved a bill making unapproved US entry a crime, and included plans to build a wall along parts of the border.
The proposal touched off mass protests in US cities. More than 500,000 people demonstrated in Los Angeles last Saturday and there were new smaller protests there by school students on Thursday.
The Senate is seeking a possible compromise that would reinforce border security but also allow work visas and eventual residency for some migrants.
The House and the Senate must agree on any new immigration rules, and the measure then has to be signed by the president to become law.
Complicating factors include the November mid-terms elections, in which opposition Democrats hope to win back seats. With several legislators hoping to run for president in 2008, there has been a growing demand for increased border security.
The senators are trying to reach a decision before a two-week recess starts on April 7.
“It’ll be a daunting task to finish this bill on that schedule,” said
Republican Senator Arlen Specter. “But we’ve undertaken daunting tasks before and succeeded, but it can be done only if we have cooperation from the members.”
Many analysts say the debate goes to the heart of what it means to be an American in a country that has long prided itself on being a nation of immigrants.
Senator Salazar himself reminded his colleagues that “virtually every person in America has a story to tell about its immigrants roots.”