A bipartisan group of senators on Tuesday unveiled long-awaited landmark legislation to remove the threat of deportation for millions of illegal immigrants and give them an opportunity to eventually become U.S. citizens.

Under the proposal, undocumented immigrants who came to America before December 31, 2011 and stayed continuously could apply for "provisional" legal status as soon as six months after the bill is signed by the president.

But beyond that, they would have to wait a decade or more without receiving federal benefits, while the government meets a host of tough conditions for securing U.S. borders and enforcing current immigration law.

The bill's sponsors - four Democrats and four Republicans - felt such conditions and enforcement "triggers" to be necessary in order to help it succeed where similar measures have failed, mostly because of opposition to what opponents see as "amnesty" for law-breakers.

Even with the many caveats, the proposal faces months of debate, scores of amendments and potentially significant opposition, particularly in the Republican-controlled House of Representatives.

Indeed, much of the legislation was designed to make the bill palatable to Republicans.

Billions of dollars in new money would be funneled into additional border security to discourage people from avoiding detection as they crossed Mexico's border with the United States.

The measure would focus on tightening porous zones in "high-risk" areas like parts of Arizona, where law enforcement has had less success in sealing the border, in part because of a more difficult terrain.

The bill sets a goal of stopping 90 percent of illegal crossings at the riskiest sections of the southern border with Mexico, either by catching people or forcing them to go back to their country.

The proposal would expand access to both low- and high-skilled labor for American businesses, attempting to keep organized labor happy with provisions designed to keep companies from hiring cheap foreign labor or filling jobs with immigrants when Americans are available.

For the U.S. technology sector, it increases the number of visas available for educated workers filling specialized jobs, though it imposes new pay requirements designed to keep the hiring from depressing wages for U.S. technology workers.

Heavy lobbying, which could complicate passage, is already underway on the visa provisions, with the construction industry, for example, unhappy with a cap placed on the number of foreigners available for construction jobs.

Still, one immigration expert who had been briefed on details of the measure before the outline was provided to reporters called it "a very smart, strategic and forward-looking bill."

For all the bill's emphasis on border control and visas, the "pathway to citizenship" remained at its heart, even though the phrase was not used in the outline made available to reporters.

Within six months from enactment, during which time the Department of Homeland Security would set forth its border security plan, the threat of deportation could end for most illegal immigrants, who would allowed to work legally in the United States after they pay an initial $500 penalty, back taxes and demonstrate that they have not been convicted of serious crime in the United States.

After 10 years - provided the government achieves control of the borders - the immigrants could apply for a "green card" or permanent resident status through an expanded merit-based immigration system.

The green card would not be automatic. A Senate aide said the majority of the 11 million illegal immigrants would likely get a green card via the merit-based visa and the total amount of penalties paid would amount to $2,000.

After that point, it could take an additional three years, after the 10-year wait for a green card, to win U.S. citizenship.

The bill was crafted by four Democratic senators: Charles Schumer of New York, Dick Durbin of Illinois, Robert Menendez of New Jersey and Michael Bennet of Colorado; and Republicans John McCain and Jeff Flake of Arizona, Lindsey Graham of South Carolina and Marco Rubio of Florida.

While Republican concerns have been muted somewhat by the clout demonstrated in the 2012 presidential election by Latino voters, the party is by no means united on immigration. Indeed, Rubio is thought to be jeopardizing any chance of one day being tapped by his party to run for president by supporting immigration reform.

The citizenship provisions have long been a goal of Democrats in Congress as well as of President Barack Obama, who has said he will submit his own immigration reform proposal should he find sufficient fault with the work of Congress.

Prospects for immigration reform were boosted immediately following the November 6, 2012 presidential elections, when Democrats held onto the White House and picked up seats in Congress.

Political analysts widely credited Hispanic Americans for some of the Democrats' success, delivering a wakeup call to Republicans who did not manage to even capture one-third of the Latino vote.

Nonetheless, plenty of Republicans will demand a tough review of the Senate bill, as well as a House bill that also is expected to be unveiled soon. Some Republicans are arguing for a piecemeal approach to immigration reform that Democrats have rejected.

(Editing by Fred Barbash and Lisa Shumaker)