WASHINGTON -- In a broad test of his executive powers, President Barack Obama said Wednesday he will sidestep Congress and unveil administrative actions on immigration — measures that could spare from deportation as many as 5 million people illegally in the U.S. and set up one the most pitched partisan confrontations of his presidency.

Obama declared that Washington has allowed the immigration problem “to fester for too long.”

The president will use an 8 p.m. EST address Thursday to announce his measures and will sign the executive actions during a rally in Las Vegas on Friday. In doing so, Obama will be taking an aggressive stand that he had once insisted was beyond his presidential power.

As many as 5 million people in the country illegally are likely to be protected from deportation and made eligible for work permits under the plan. They would not have a path to citizenship, however, and the actions could be reversed by a new president in two years. Officials said the eligible immigrants would not be entitled to federal benefits — including health care tax credits — under Obama’s plan.

The 5 million estimate includes extending deportation protections to parents and spouses of U.S. citizens and permanent residents who have been in the country for some years. The president also is likely to expand his 2-year-old program that protects young immigrants from deportation. The administration had considered extending the executive action to parents of young immigrants covered under a 2012 Obama directive but immigration advocates said they did not expect them to be included in the final plan.

“What I’m going to be laying out is the things that I can do with my lawful authority as president to make the system better, even as I continue to work with Congress and encourage them to get a bipartisan, comprehensive bill that can solve the entire problem,” Obama said in a video on Facebook.

Laying the groundwork for his actions, Obama invited 18 Democratic members of the House and Senate — but no Republicans — to dinner at the White House Wednesday.

Obama is to speak at Las Vegas’ Del Sol High School on Friday, a school with a large population of non-English speaking students where Obama unveiled his blueprint for comprehensive immigration legislation in 2013.

Republicans vehemently oppose the president’s likely actions, but are deeply divided and have spent much of the week intensely debating how to respond. Some conservative members have threatened to pursue a government shutdown and one — two-term Republican Rep. Mo Brooks of Alabama — raised the specter of impeachment on Wednesday.

House Speaker John Boehner’s spokesman criticized Obama’s planned announcement, noting that the president himself has said in the past that he is not “emperor” and is limited in his ability to act on his own.

“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,” the spokesman, Michael Steel, said.

A wide-ranging immigration bill passed the Senate last year, but stalled in the Republican-led House. Senate Democratic leaders on Wednesday took turns declaring their support for Obama’s unilateral action, blaming Republican inaction for forcing Obama to act.

“There’s one more chance: just put the bill on the floor Speaker Boehner,” said Sen. Chuck Schumer, D-N.Y., a lead author of the comprehensive immigration bill which passed the Senate last year and stalled in the House. “Pass the bill and we will not even have to debate executive action.”

Even Republicans who supported the Senate bill that overhauled immigration laws said Obama’s go-it-alone approach would backfire, but they also cautioned their own party colleagues not to overreach in their response.

Sen. Lindsey Graham, R-S.C., who worked on the Senate legislation, said the executive actions would leave the status of millions of immigrants unresolved and would not address what he called a broken immigration system.

“Our response has to be measured — can’t capitulate, can’t overreact,” he said. “Impeachment or shutting down the entire government would be an unwise move.”

Adjustments also are expected to the 2012 Deferred Action for Childhood Arrivals program that allowed immigrants under 31 who had arrived before June 2007 to apply for a reprieve from deportation and a work permit. More than 600,000 young immigrants have been shielded from deportation to date under the program. Removing the upper age limit so that applicants don’t have to be under 31 — one option under consideration — would make an additional 200,000 people eligible.

The beneficiaries of Obama’s new executive action would be treated in the same manner as those immigrants who were shielded from deportation in Obama’s 2012 directive, according to one official who discussed the limits of Obama’s action on the condition of anonymity, lacking authority to speak on the record at this point.

Those young immigrants covered by the 2012 action can obtain work permits but are not eligible for food stamps, federal welfare benefits or disability benefits under the Supplemental Security Income program. They also are ineligible for tax credits under Obama’s health care law, though they can buy health coverage at full price on the exchanges created by the law. They may be eligible for public benefits provided by some states, however.

Some immigrant advocates worried that even though Obama’s actions would make millions eligible for work permits, not all would participate out of fear that Republicans or a new president would reverse Obama’s actions.

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Associated Press writers Julie Pace, Donna Cassata, and Alicia Caldwell contributed to this report.