WASHINGTON, Jan 29 (Reuters) - President Barack Obama will wade cautiously into the debate over U.S. immigration reform on Tuesday, seeking to build momentum for a new bipartisan plan to offer a pathway to citizenship for the country's 11 million illegal immigrants.

Reflecting the growing clout of Hispanic voters, Obama will travel to Nevada little more than a week after his second inauguration and make the case for swift action by Congress to overhaul immigration laws.

Immigration reform could give Obama a landmark second-term legislative achievement, but he is expected to tread carefully in a speech in Las Vegas, just a day after a group of influential Senate Democrats and Republicans laid out a broad plan of their own.

Obama's challenge is to help build public support for the senators' framework, which is in line with many of his main ideas for a sweeping immigration overhaul, while not alienating his fiercest Republican foes who might resist anything with the Democratic president's name on it.

While Obama is likely to use the bully pulpit of the presidency, backed up by a White House-organized grass-roots campaign, he will likely be more circumspect for now about how personally involved he becomes in congressional negotiations.

"The minute it becomes Obama's plan, the Republicans kick automatically into opposition," said Bill Schneider, a political scientist at George Mason University in Virginia. "The White House knows to back off for now."

Scheduled to speak at a Las Vegas high school at 11:15 a.m. PST (1915 GMT), Obama does not intend to unveil legislation of his own. He will instead urge lawmakers to press ahead with their efforts even as he restates the "blueprint" for reform he rolled out in 2011, which called for an "earned" path to citizenship, administration officials said.

The flurry of activity marks the first substantive drive in years to forge an agreement on fixing America's flawed immigration system. Though the debate is likely to be contentious there is a growing consensus in Washington that the conditions are finally ripe for tackling the problem.

Obama and his fellow Democrats see their commitment to immigration reform as a way to solidify their hold on the growing Latino vote, which they won handily in the 2012 election. Nevada, for example, has a fast-growing Hispanic population that helped Obama carry the state in the November election.

Many Republicans, worried that their party has alienated Hispanics with anti-immigrant rhetoric, are suddenly open to cooperation on the issue as they seek to set a new tone.

DEVIL IN THE DETAILS?

The eight-member Senate group includes John McCain, a Republican from the border state of Arizona; Charles Schumer, a centrist Democrat from New York; and Republican Marco Rubio of Florida, a Cuban-American favorite of the Tea Party movement who has helped garner support from influential conservatives.

Translating the aspirations expressed by the group into an inevitably lengthy and complicated bill will itself be a major challenge in Congress. At the same time, the White House wants to see further details before Obama will fully embrace the senators' approach.

In an attempt to build support, the Senate proposal would couple immigration reform with enhanced border security efforts aimed at preventing illegal immigration and ensuring that those foreigners temporarily in the United States return home when their visas expire.

Under the proposal, undocumented immigrants would be allowed to register with the government, pay a fine, and then be given probationary legal status allowing them to work.

Ultimately, these immigrants would have to "go to the end of the line" and apply for permanent status. But while waiting to qualify for citizenship, they would no longer face the fear of deportation or harassment from law enforcers if they have steered clear of illegal activity.

Obama's aides consider it a breakthrough that Republican members of the bipartisan group of senators have agreed to a path to citizenship, a concept that many in their party have long opposed as tantamount to amnesty for law-breakers.

The White House remains wary, however. The president's aides have written up extensive legislative language for an immigration overhaul and will step in with their own formal proposals if the Senate effort falls apart, an administration official said.

Immigration reform, sidelined by economic issues and healthcare reform during Obama's first term, is part of an ambitious liberal agenda he laid out in his second inaugural address. That agenda also includes gun control, gay rights and fighting climate change.

Last summer, Obama took executive action so that the federal government stopped seeking to deport illegal immigrants who had arrived in the United States as children - a dramatic change that was celebrated in the Hispanic community.

After winning the bitterly fought election, Obama promised to tackle the issue comprehensively early in his new term.

(Editing by Alistair Bell and Christopher Wilson)