WASHINGTON (CNN) -- President Barack Obama tried Tuesday to build momentum for an immigration reform bill facing a key Senate procedural vote, saying the measure was a commonsense approach to fix a broken system that has allowed 11 million people to live illegally in America.
At a White House event a few hours before the Senate was scheduled to vote on whether to debate the compromise measure, Obama emphasized that it would increase spending on border security and require undocumented immigrants to pursue what he called an "arduous" path to eventual citizenship.
"You have to pass background checks, you have to learn English, you have to pay taxes and a penalty and then you have to go to the back of the line behind everybody who has done things the right way and have tried to come here legally," Obama said in addressing a major complaint by Republican opponents who call the reform measure an amnesty.
The legislation addresses an emotionally charged issue with huge political stakes for both parties. However, fierce opposition by conservatives may prevent it from getting through Congress despite years of negotiation and major compromise.
It aims to create a 13-year path to citizenship for most of the country's undocumented immigrants, a bipartisan proposal hammered out by the so-called "Gang of Eight" senators this spring that has the backing of Obama.
Polls show many Americans also favor some form of immigration policy overhaul, depending on the details of legislation.
GOP leaders have signaled their support for starting debate, so a procedural motion aimed at doing so is expected to receive the 60 votes necessary to take that next step even if the fate of the legislation is uncertain.
Obama and congressional Democrats are anxious to fulfill a long-delayed pledge to Latino voters to pass reforms to the troubled immigration system. Passage could pay political dividends for their party for many years.
Republicans are in a more difficult bind.
Latinos backed Obama over Mitt Romney by a 44-point margin in November and GOP strategists are concerned about the party's long term viability in national elections if that trend is not reversed.
Some congressional conservatives say opposing the "Gang of Eight" plan is a matter of principle and they won't bend. They remain skeptical about any measure offering a path to citizenship. A lot of them consider it amnesty.
That provision, coupled with concerns about whether the bill really will tighten security along the nation's porous borders as proposed, may make it difficult for conservatives to support -- especially those up for re-election next year.
"The bill grants permanent legal status to millions of undocumented immigrants, as currently written, without really any guarantee of securing the border. Now, how would that possibly be a good idea?" asks Sen. John Cornyn of Texas, the chamber's No. 2 Republican.
Sen. Marco Rubio, R-Florida, a member of the bipartisan "Gang of Eight," is uniquely positioned to influence whether a large number of Republicans will support the bill.
Senate passage with 70-plus votes could give it much-needed momentum in the House. If Republicans aren't persuaded to support the bill, it might limp out of the Senate with mostly Democratic backing, giving little reason for House GOP leaders to push for a vote.
Rubio is a popular conservative who may run for president in 2016. He has made clear that border security requirements must be toughened if he and other GOP skeptics will support it.
He is pushing an amendment that would make Congress, not the executive branch, responsible for deciding if security metrics at the border have been met before other aspects of the reform bill -- including the pathway to citizenship -- are triggered.
Because Rubio's support is so critical, other members of the bipartisan group have hinted they likely will back his proposal even though they are reluctant to make major changes to their original compromise.
The vote on the Rubio amendment will be one of the most closely watched as floor action unfolds over the next three weeks.
It's not clear when that vote will take place. But there will be many other amendments related to stepped up security checks of people entering the country, temporary visas for high and low skilled workers, and other technical provisions.
Republican opponents are expected to offer amendments to undermine the plan and weaken support.
"Gang of Eight" members have vowed to work together to defeat those amendments and prevent key parts of the agreement from crumbling.
"Senators will propose a number of ideas to make the legislation better. Some will offer ideas to make it worse. But those suggestions must preserve the heart of the bill," said Senate Majority Leader Harry Reid, D-Nevada.
Advocates for comprehensive reform won the first major legislative victory last month when the Senate Judiciary Committee voted 13-5 to approve the "Gang of Eight" plan.
Democratic leaders hope to have a vote on final passage by the end of June.
A bipartisan group has been working on an immigration plan in the House but the effort suffered a setback last week when a key member dropped out of the negotiations.
Previous efforts to pass immigration reform fell short last decade even though it was said to be a priority of President George W. Bush at the time.
CNN's Tom Cohen contributed to this report.