Washington - President Barack Obama will announce his intention today to use his executive powers to shield millions of illegal immigrants from deportation, the most significant presidential intervention to alter the U.S. immigration system in nearly three decades.

Using a rare prime-time address from the White House, Obama hopes to build public support for his decision to remake border control laws without approval from Congress, a move likely to spark a major political fight about the limits of presidential power. Obama will provide more details at a rally with immigration advocates in Las Vegas on Friday as the White House begins a campaign to mobilize grass-roots support for the effort.

Obama is expected to announce measures that would make as many as 4 million immigrants eligible for temporary protected status and provide relief to another 1 million through other means, according to people briefed on the plans. He also will take steps to expand visas for high-tech workers; modify federal immigrant detention procedures; and add resources to strengthen security at the border, they said.

"Everybody agrees that our immigration system is broken," Obama said in a brief video released on his Facebook page Wednesday. "Unfortunately, Washington has allowed the problem to fester for too long."

But even as he moved to address an old problem, Obama created a new political tempest on Capitol Hill, where Republicans quickly denounced his actions less than two weeks after voters handed the GOP full control Congress.

A spokesman for House Speaker John Boehner, R-Ohio, referred to the president as "Emperor Obama" and charged that he was exceeding his constitutional authority and cementing "his legacy of lawlessness." Several GOP members suggested using a government spending bill next month to counter Obama's move by defunding related immigration programs, setting up the potential for a government shutdown.

"This is presidential overreach of monumental proportions," said Sen. Jeff Sessions, R-Ala., the incoming chairman of the Senate Budget Committee. "It's a direct erosion of congressional power. . . . I think Congress needs to and will resist and do so in a way that is responsible."

The showdown comes two years after Obama, flush off his 2012 re-election, announced he would pursue a legislative overhaul of immigration laws as he sought to fulfill a campaign promise to Latino and Asian American supporters who were frustrated that the president had not done more on immigration in his first term.

But an 18-month effort to pass legislation, which included a path to citizenship, collapsed this summer in the face of strenuous opposition in the Republican-controlled House. In June, Obama promised to act aggressively on the problem in areas where the law allowed the use of his executive authority.

Administration lawyers have spent months reviewing case law and meeting with immigration advocates, law enforcement officials and legal experts to develop options for Obama, who reviewed them with Department of Homeland Security Secretary Jeh Johnson and Attorney General Eric Holder Jr.

Under his plan, Obama will provide administrative relief to illegal-immigrant parents of children who are U.S. citizens or legal permanent residents, an adult population that reaches an estimated 3.46 million, according to a report from the Migration Policy Institute. However, the president's order also will require the parents to have lived in the United States for a certain number of years - likely 10. The average illegal immigrant has lived in the country for 13 years.

Many of those who receive administrative relief also would be eligible to apply for work permits.

The president also is expected to expand a 2012 program that has deferred the deportations of nearly 600,000 younger immigrants known as "dreamers" who were brought into the country illegally as children. Obama's plans would expand that program by raising the maximum current age from 30 and raising the maximum arrival age above 16. However, it is not known how many years of eligibility he will add at either end or how many more people would be covered.

"I am getting slammed with calls from people asking me if they will qualify, but I can't tell them anything yet," said Simon Sandoval Moshenberg of the Legal Aid Justice Center in Falls Church, Va., which helped scores of young immigrants apply for the deferred action program over the past two years. "Any time you draw a line, there are people who fall on either side. It was heartbreaking to see people miss the age cutoff date by just a few weeks, and that can happen again."

Obama will stop short of extending protections to hundreds of thousands of parents of the dreamers, after the White House Office of Legal Counsel determined that doing so would exceed the president's legal authorities to act unilaterally, according to the people briefed on the plans.

About 671,000 parents live with illegal-immigrant children, and 6.5 million live with no children at all - meaning that most of them are unlikely to be eligible for deportation relief, although some might qualify under other potential changes, including if they serve in the U.S. military.

"Even if they help 5 million people, a lot of others will be left out, but it is still a huge step forward and an historic decision. It is one of the biggest victories we've ever had," said Frank Sharry, executive director of America's Voice, a national immigrant advocacy group based in Washington.

Despite its limits, Sharry said Obama's executive action "could well be a turning point from a decade of ramping up enforcement to a more balanced approach, where protection of immigrants is combined with enforcement against bad actors."

White House officials declined to provide details about the president's announcement, but Johnson called the president's plan "comprehensive." He and other administration officials emphasized that Obama remains supportive of Congress' undertaking a broader legislative fix to immigration laws next year.

"Legislative action is always preferable," Johnson said during an appearance at the National Press Club on Wednesday. "But we have waited for Congress to act, and the Congress has not acted. It can't be that we're not allowed to lift a finger to fix our broken immigration system. And we will."

For Republicans, the confrontation represents the first test of GOP leaders' ability to calibrate their political strategy after the party's successes in the midterm elections. While some members appear eager for a fight with the White House, others cautioned that the party should tread carefully, underscoring the mounting concerns among Republican officials over the simmering outrage of conservative lawmakers and its political consequences, particularly with the Hispanic community.

Sen. Mitch McConnell, R-Ky., the soon-to-be majority leader, expressed confidence Wednesday that congressional Republicans will extend the federal government's funding, which is set to expire next month.

"Republicans have to handle this in a sensitive way," said Sen. Orrin Hatch, R-Utah. He warned that any Republican who speaks of impeaching the president over immigration would be shunned by most Republicans and "considered pretty erratic and pretty excessive."

In an effort to cast doubt on the legality of Obama's decision, GOP aides circulated a list of 22 times the president has said publicly that he doesn't believe he has the authority to act through executive action to change immigration laws.

Two Republicans mulling presidential bids, Sens. Rand Paul of Kentucky and Rob Portman of Ohio, urged colleagues to focus on fighting the policy rather than on fiery rebuttals that could alienate the voters the party is seeking to win over ahead of the 2016 campaign.

"Pass some things," Paul said, when asked about how the party should react. He suggested an expansion of work visas as an example of one potential piece of legislation that could highlight Republicans' commitment to addressing immigration issues and to governing. "We can still do some good things. I think we have to fight him that way."

The White House, recognizing the political risks, moved to shore up public support with an aggressive sales job. By Wednesday afternoon, Obama's Facebook video had been watched more than 1.7 million times.

His prime-time address tonight also is aimed to include "as many people across country in this broader debate," White House press secretary Josh Earnest said. However, most of the major television networks announced they will not carry the remarks live, leaving it to the cable news channels and the Spanish-language network Univision, where Obama's remarks will coincide with the airing of the Latin Grammy Awards.

On Friday, Obama will return to Las Vegas' Del Sol High School, which holds a special significance for him. On Jan. 11, 2008, then-Sen. Obama arrived in that city after a humiliating defeat to then-Sen. Hillary Rodham Clinton in New Hampshire's presidential primary. Obama pivoted to the heavily Latino voting bloc of Nevadans and delivered what became known as his "Si Se Puede!" speech-the Spanish equivalent of "Yes, We Can!"

At the high school that day, hundreds filled the gymnasium, and hundreds more waited outside, as the president vowed to deliver on citizenship for undocumented immigrants.