U.S. President Barack Obama speaks at a news conference at the end of the G20 summit in Brisbane November 16, 2014. REUTERS/Jason Reed

By Steve Holland and Richard Cowan

WASHINGTON (Reuters) - President Barack Obama will outline a plan on Thursday to relax U.S. immigration policy for as many as 5 million people, bypassing Congress and angering Republicans.

U.S. Representative Paul Ryan, the leading Republican voice on fiscal policy and a potential 2016 presidential candidate, called the plan a "partisan bomb" while a spokesman for House Speaker John Boehner branded the president “Emperor Obama” for acting unilaterally.

The White House said Obama will deliver a televised speech at 8 p.m. ET on Thursday (0100 GMT Friday) laying out the plan followed by a trip to Las Vegas on Friday. Nevada is home to the highest proportion of undocumented immigrants.

Frustrated by years of congressional inaction on what most in Washington agree is a broken immigration system, Obama said he is now prepared to use his executive authority.

Obama's directives are expected to remove the threat of deportation for as many as 5 million of the estimated 11 million people living illegally in the United States.

The decision will cement his legacy as having aided Hispanics who helped elect him in 2008 and who have become increasingly vocal in their frustration that he has failed to live up to his promise to enact immigration reform.

The unilateral overhaul will likely have a ripple effect on the campaign to find a successor to the president in 2016. While Hispanics will no doubt be pleased, Democrats could face a backlash from voters.

Reaction was swift from Republicans who took control of the Senate in Nov. 4 elections and strengthened their grip on the House of Representatives.

While liberal Democrats were thrilled at Obama's decision to move ahead, some moderate voices in Obama's party were uneasy.

"I wish he wouldn't do it," said Democratic Senator Joe Manchin of West Virginia. "I just wish he wouldn't do it."

Obama will host 18 congressional Democrats at the White House to consolidate support for his immigration plans among his closest allies on Capitol Hill.

Some conservative Republicans have threatened to fight the immigration move by imposing funding restrictions in a must-pass spending bill, which could conceivably lead to a government shutdown.

Republican leaders, however, have stressed they will not allow a shutdown after facing heavy criticism for the last one a year ago.

House Republicans are weighing a range of responses to register their disapproval.

"If 'Emperor Obama' ignores the American people and announces an amnesty plan that he himself has said over and over again exceeds his constitutional authority, he will cement his legacy of lawlessness and ruin the chances for congressional action on this issue and many others," said Michael Steel, spokesman for House Speaker John Boehner.

For a president known for having deported thousands of illegal migrants, the actions he will take mark a dramatic shift in course, although advocacy groups will argue that he should go even further in protecting more people who work low-paying jobs that many American citizens prefer not to do.

Sources close to the administration said Obama is planning to issue a reprieve from deportation that will cover some parents of U.S. citizens and legal permanent residents.

That initiative would expand on a 2012 executive order by the president that gave relief from deportation and work permits to undocumented children brought to the United States by their parents.

There is also expected to be a border security element and Obama will act to help companies hire and retain high-skilled workers from abroad, the sources said.

Obama's move is his most defiant step yet in reaction to the elections handing control of the Senate to Republicans. The new political order in Washington will test Obama's ability to make compromises with his opponents.

An NBC News/Wall Street Journal poll found 48 percent would prefer Obama not act on his own, while 38 percent support it and 14 percent had no opinion or were unsure.

The last major immigration overhaul that expanded the number of legal migrants was in 1986 through legislation signed by Republican President Ronald Reagan. An attempt by President George W. Bush in 2007 for immigration reform failed.

It is not out of the question that Obama early next year could offer to approve the long-stalled Keystone XL pipeline from Canada in exchange for a deal on immigration legislation.

Despite claims that Obama may overstep his executive powers, Stephen Legomsky, a former U.S. immigration official who is now a professor at Washington University law school, said the president's planned action appeared to fit well within the bounds of established law and prosecutorial discretion.

(Additional reporting by Julia Edwards, Susan Cornwell, Doina Chiacu, Amanda Becker, Susan Heavey and Roberta Rampton; Editing by Tom Brown)