WASHINGTON -- Key Democratic and Republican senators pledged Monday to propel wide-ranging immigration legislation through the U.S. Senate by summer, even as they acknowledged pitfalls ahead that could doom their effort just as previous attempts have failed.

The bipartisan group unveiled proposals to provide a pathway to citizenship for 11 million illegal immigrants already in the U.S. while beefing up border security, allowing more temporary workers and cracking down on employers who would hire illegal immigrants. The plans were admittedly short on detail and all present acknowledged that months of tedious and politically treacherous negotiations are ahead before they can claim success.

But with a newly re-elected President Barack Obama also pledging his commitment, the lawmakers argued that six years after the last sustained congressional effort at immigration reform came up short in the Senate, this time would be different.

"Other bipartisan groups of Senators have stood in the same spot before, trumpeting similar proposals," said Sen. Charles Schumer, a Democrat. "But we believe this will be the year Congress finally gets it done. The politics on this issue have been turned upside down," Schumer said, arguing that polls showed more support than ever for immigration reform and political risk in opposing it.

Pressures from outside groups from business to organized labour to immigrants themselves will be immense, even as lawmakers warily eye voters for their reaction. Passage of legislation by the full Democratic-controlled Senate is far from assured, but the tallest hurdle could come in the House, which is dominated by conservative Republicans who've shown little interest in immigration reform.

Many Republicans feel they have to take a tough stance on illegal immigration to ward off challengers in primary elections from more conservative members of the party.

"Elections. Elections," said Sen. John McCain, a Republican. "The Republican Party is losing the support of our Hispanic citizens. And we realize that there are many issues on which we think we are in agreement with our Hispanic citizens, but this is a pre-eminent issue with those citizens."

Obama got 71 per cent of the Latino vote in November compared to 27 per cent for Republican Mitt Romney, who won the Republican nomination in part for his tough stand on illegal immigration. The president travels to the western state of Nevada Tuesday to lay out his proposals for immigration reform which are expected to be similar to the Senate proposals in many ways.

In the past Republicans have taken a hard line on illegal immigration, but the election result is causing some of them to shift their opinion. Hispanics are a growing bloc and likely to become more important in future elections.

In a five-page framework the lawmakers set out plans for creating a path to citizenship for illegal immigrants already here, contingent upon securing the border and better tracking of people here on visas; reforming the legal immigration system, including awarding green cards to immigrants who obtain certain advanced degrees from American universities; creating an effective high-tech employment verification system to ensure that employers do not hire illegal immigrants in the future; and allowing more low-skill and agricultural workers.

In a sign of the challenges ahead, the proposals immediately got a cool reaction from Senate Republican Leader Mitch McConnell. "This effort is too important to be written in a back room and sent to the floor with a take-it-or-leave it approach," McConnell said. "It needs to be done on a bipartisan basis and include ideas from both sides of the aisle."

Sen. Jeff Sessions, a Republican, said on the Senate floor that "no one should expect members of the Senate are just going to rubberstamp what a group has met and decided."

Senate Majority Leader Harry Reid, a Democrat, applauded the framework and said, "I will do everything in my power to get a bill across the finish line."

Besides McCain and Schumer, the senators endorsing the new principles Monday were Democrats, Dick Durbin of Illinois, Robert Menendez of New Jersey and Michael Bennet of Colorado; and Republicans Lindsey Graham of South Carolina, Marco Rubio of Florida and Jeff Flake of Arizona.

Several of these lawmakers have worked for years on the issue. McCain collaborated with the late Democratic Sen. Edward M. Kennedy on the comprehensive immigration legislation pushed by then-President George W. Bush that failed in 2007.

The group claims a notable newcomer in Rubio, a potential 2016 presidential candidate whose conservative bona fides may help smooth the way for support among conservatives wary of anything that smacks of amnesty. Rubio has been working with the group while also detailing his own, very similar immigration proposals to selected media, getting a generally positive reaction from conservative media hosts.

"There are 11 million human beings in this country today that are undocumented. That's not something that anyone is happy about, that's not something that anyone wanted to see happen, but that is what happened. And we have an obligation and the need to address the reality of the situation that we face," Rubio said.

As the group turns to the tough work of writing legislation, which they hope to see come to a vote in the Senate Judiciary Committee in March, there may be most disagreement over the path to citizenship. In order to satisfy the concerns of Rubio and other Republicans, the senators are calling for the completion of steps on border security and oversight of those here on visas before taking major steps forward on the path to citizenship.

Even then, those here illegally would have to pass background checks and pay fines and taxes in order to qualify for a "probationary legal status" that would allow them to live and work here -- but not qualify for federal benefits -- before being able to apply for permanent residency, a critical step toward citizenship. Once they are allowed to apply they would do so behind everyone else already in line for a green card within the current immigration system.

That could be a highly cumbersome process, but how to make it more workable is being left to future negotiations. The senators envision a more streamlined process toward citizenship for immigrants brought here as children, and for agricultural workers.