* U.S. senators back broad outline for legislation
* Tough fight seen for first major bill since 1986
* Seeks smoother flow of immigrants to serve workforce needs (Rewrites with more reaction)
By Richard Cowan and Rachelle Younglai
WASHINGTON, Jan 28 (Reuters) - Reflecting the growing influence of Hispanics in American politics, a group of Republican and Democratic U.S. senators on Monday said they will move quickly with legislation giving 11 million illegal immigrants a chance to become citizens.
That new political reality, and a change in public opinion on immigration, makes this effort at comprehensive reform more likely to succeed than those of the past, they said, acknowledging that it still won't be easy.
"The Republican Party is losing support of our Hispanic citizens," said John McCain, a Republican from the border state of Arizona and one of the eight senators working on the initiative.
Another member of the bipartisan group, Democrat Charles Schumer, said the public's attitude has changed. "Four years ago they said 'fix the border.' Now they say they much prefer a comprehensive solution including a path to citizenship as well as fixing the border," the New York senator said.
In a Congress unaccustomed to Republicans and Democrats working together on anything, the bipartisan sponsorship alone was enough to elevate the proposal on Washington's agenda and generate excitement among advocates around the country.
"We're hopeful because the Republicans are meeting with Democrats," said Georgina Sanchez, an undocumented business owner in Phoenix, who was among a few activists who spent all night at a vigil for humane immigration outside the Arizona Capitol building. "It gives us a lot of hope."
The drive may pick up momentum Tuesday when President Barack Obama, in a visit to Las Vegas, lays out a vision similar to that outlined Monday by the Senators. Obama does not intend to unveil a new immigration plan of his own, an administration official said, beyond restating the "blueprint" for reform he rolled out in 2011, which called for an "earned" path to citizenship, with some additional details.
Whether the hopes will founder once details are added to the outline, remains to be seen. Translating the aspirations expressed by the bipartisan group into an inevitably lengthy and complicated bill will itself be a major challenge.
In an attempt to build support among lawmakers, the Senate proposal would couple immigration reform with enhanced security efforts aimed at preventing illegal immigration and ensuring that those foreigners here temporarily 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 after arriving in the United States.
Missing were potentially controversial estimates about the future flow of immigrants under the legislation and the timeline undocumented immigrants would face for winning citizenship.
In light of growing Hispanic electoral power - and with U.S. business demanding reforms to help them fill employment needs - leading Republicans have been urging conservatives to rethink both their positions and their rhetoric.
The Senate group included Republican Senator Marco Rubio of Florida, a favorite of the Tea Party movement who has helped garner support from influential conservatives.
Schumer said he hoped the Senate could pass legislation in late spring or early summer.
A bipartisan group in the House of Representatives also is close to unveiling its own immigration proposals, according to a congressional source with knowledge of the reform efforts. That is likely before Obama's Feb. 12 State of the Union address to a joint session of Congress.
AN UPHILL BATTLE
Republican Representative Mario Diaz-Balart of Florida, a conservative who has taken a strong interest in immigration reform, praised the Senate outline.
Asked whether he thought a reform bill could pass the House, Diaz-Balart said: "I think we have a legitimate chance of presenting and passing legislation that will pass both the House and the Senate. It is, in a big way, an uphill battle."
Indeed, Republican Senator Jeff Sessions of Alabama, an opponent of past legislative efforts, on Monday delivered a Senate floor speech in which he acknowledged the need for reforms.
But he repeatedly complained that current immigration laws were not being enforced, especially ones prohibiting immigrants who need federal aid once they arrive in the United States.
"A large-scale amnesty is likely to add trillions of dollars to the debt over time," Sessions said.
James Carafano, an immigration and border security expert at the Heritage Foundation, which carries influence with Republicans, expressed concerns that the new initiative could be much like 2007 reform legislation "all over again."
He added that the earlier measure would have failed to fix structural problems in immigration by not effectively dealing with illegal border crossings over the long run.
The last comprehensive revision of the nation's immigration law was in 1986. Numerous efforts since then have encountered stiff resistance, especially from the conservative wing of the Republican Party, which as recently as the Republican presidential primary races in 2012 opposed anything resembling an "amnesty" for undocumented immigrants.
"When you legalize those who are in the country illegally, it costs taxpayers millions of dollars, costs American workers thousands of jobs and encourages more illegal immigration," warned Republican Representative Lamar Smith, who is the former chairman of the House Judiciary Committee.
The White House praised the group's efforts but warned that Obama would not be satisfied until there was meaningful reform. The president "will continue to urge Congress to act until that is achieved," a White House spokesman said. (Additional reporting by Jeff Mason in Washington and Tim Gaynor in Arizona; Editing by Fred Barbash, Doina Chiacu and Lisa Shumaker)