WASHINGTON - 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 US and set up one the most pitched partisan confrontations of his presidency.

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 House lawmaker raised the specter of impeachment on Wednesday.

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

The president will use a primetime 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.

Obama has said executive action is necessary because a wide-ranging immigration bill passed the Senate last year, but stalled in the Republican-led House.

Republicans have warned that unilateral action by the president would only complicate prospects of Congress passing a comprehensive immigration bill and endanger cooperation on other issues during Obama's final two years in office. After sweeping midterm elections this month, the Republicans will control both the Senate and the House when the new Congress takes over in January.

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 US 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.

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.

Among the networks airing his Thursday speech will be Spanish-language Univision, which will interrupt the Latin Grammys to carry his remarks, assuring him a huge Spanish-speaking audience.

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.

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, lead author of the comprehensive immigration bill that passed the Senate last year. "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, 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."