The US Senate has voted to begin debate and amendments on a historic immigration bill, burying a procedural roadblock that opponents regularly use to delay or even kill legislation.
November's election results indicate broad support for updating the country's immigration laws.
Some senators who have expressed opposition to the Senate bill voted to allow the debate to go ahead.
By a vote of 82-15, the Senate cleared the way for the long-anticipated debate that could extend through June.
Opponents of the bill quickly offered amendments to significantly change or possibly kill the measure if adopted.
Republican Senator Charles Grassley of Iowa introduced a plan to certify "effective control over the entire southern border" for a period of six months before any of the 11m undocumented residents in the US could begin applying for legal status.
Other Republican senators are pushing similar proposals.
The legalisation and ultimate citizenship for the 11m is a central component of the bill.
US Democrats and some Republicans have vowed to block any measure that leaves their fate in doubt indefinitely.
Earlier today, President Barack Obama sought to inject momentum into the push for US immigration reform.
Mr Obama said: "If you genuinely believe we need to fix our broken immigration system, there's no good reason to stand in the way of this bill."
President Obama, who won re-election last year thanks in part to strong support from Latino voters, has made immigration reform a top priority of his second term.
He had not given a major public address on the issue for some time, reflecting a White House strategy of not wanting to get in the way of the bipartisan bill's progress in the Senate.
The Senate bill would authorise billions of dollars in new spending for enhanced border security.
It would create new visa programmes for high and low-skilled workers.
The bill if passed would provide a pathway to citizenship for roughly 11 million undocumented immigrants.
Many of the immigrants are from Mexico, Central America and approximately 50,000 Irish currently are living illegally in the country.
As US congress plunged into a contentious debate on the bill, Senator Tim Kaine of Virginia, a Democrat, delivered a Senate speech in support of the bill in Spanish.
Senate officials said it was the first time in at least decades that a floor speech was spoken entirely in a language other than English.
The bill will need backing from some Republicans in order to give it momentum in the more conservative, Republican-controlled House of Representatives, where the pathway to citizenship provisions face deeper scepticism.
Four Republicans joined with four Democrats in writing the Senate bill earlier this year.
In a sign of the hurdles to come, House Speaker John Boehner of Ohio said he expected immigration reform to be law by the end of the year.
But he said the Senate measures to enforce the changes and secure the US border with Mexico were inadequate.
And Senate Republican leader Mitch McConnell of Kentucky warned in a speech in the Senate: "In days ahead there will be major changes in this bill if it is to become law."
Immigrant groups fear that too many changes could erode a delicate coalition now pushing the bill.