The White House is seen at dusk in Washington, Wednesday, Nov. 19, 2014. President Barack Obama on Thursday is expected to announce steps he will take to shield up to 5 million immigrants illegally in the United States from deportation. (AP Photo/Jacquelyn Martin)

WASHINGTON (AP) — As broadly as President Barack Obama may push the limits of his authority to shield from deportation millions of immigrants illegally in the United States, the fate of millions more will still be left unresolved.

Obama is preparing to flex his executive powers Thursday, using an 8 p.m. EST address to announce that he is sidestepping Congress and ordering his own federal action on immigration. Incoming Senate Majority Leader Mitch McConnell promised Thursday to thwart his plan.

The reaction from congressional Republicans had been swift and fierce even before McConnell took to the Senate floor, hours before the president’s speech, to warn that “Congress will act” to stop him.

Obama’s measures could make as many as 5 million people eligible for work permits, with the broadest action likely aimed at extending deportation protections to parents of U.S. citizens and permanent residents, as long as those parents have been in the country for five years.

Other potential winners under Obama’s actions would be young immigrants who entered the country illegally as children but do not now qualify under a 2012 directive from the president.

With more than 11 million immigrants living in the country illegally, Obama’s actions would still leave millions unprotected even though their chances of getting deported if they have not committed a crime are low.

“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 posted Wednesday on Facebook.

McConnell pushed back hard on Thursday morning.

“We’re considering a variety of options,” he told Senate colleagues. “But make no mistake. When the newly elected representatives of the people take their seats, they will act.”

“The president will come to regret the chapter history writes if he does move forward,” the Kentucky Republican said.

Obama nevertheless was ready to sidestep Congress and order his own federal action.

The president is expected to extend temporary relief from deportation, along with work permits, to as many as 5 million immigrants now in the country illegally. The action would affect nearly half the around 11 million who are here illegally, and would apply to those who’ve been in the country at least five years and have close family ties to U.S. citizens.

But the vehement reaction of Republicans, who will have complete control of Congress come January, made clear that Obama was courting what could be one of the most pitched partisan confrontations of his presidency.

How Republicans will respond remained uncertain, and the party was divided. But a major battle on Capitol Hill looked inevitable.

Some on the right pushed for using must-pass spending legislation to try to shut-down Obama’s move. One lawmaker— two-term Republican Rep. Mo Brooks of Alabama — raised the specter of impeachment Wednesday.

Party leaders warned against such talk and sought to avoid spending-bill tactics that could lead to a government shutdown. They said such moves could backfire, alienating Hispanic voters and others.

In a closed-door meeting with Senate Republicans, McConnell urged restraint. Still, there were concerns among some Republicans that the potential 2016 presidential candidates in the Senate would use the announcement to elevate their standing, challenging Obama directly. Conservatives in the House could be even harder to control

Obama was poised to announce an executive action that would make parents of U.S. citizens and permanent residents eligible for work permits would affect about 3.3 million immigrants if it requires that they have lived in the U.S. for five years, according to the Migration Policy Institute. If the action includes spouses of U.S. citizens and permanent residents, the number of eligible immigrants rises to 3.8 million.

The president also is likely to expand his 2-year-old program that allowed immigrants under 31 who had arrived before June 2007 to apply for a reprieve from deportation and a work permit — a program that to date has shielded more than 600,000 young immigrants from deportation. One option under consideration would remove the upper age limit so applicants don’t have to be under 31.

Obama’s steps, however, would not include the parents of those young immigrants — a move many advocates had vigorously encouraged him to take. He also was not including special protections for farm workers sought by the United Farm Workers, though the provisions in his plan would allow up to 250,000 farm workers to be eligible for work permits, according to Giev Kashkooli, the UFW’s national political legislative director.

As far-reaching as Obama’s steps would be, they fall far short of what a comprehensive immigration overhaul passed by the Senate last year would have accomplished. The House never voted on that legislation. It would have set tougher border security standards, increased caps for visas for foreign high-skilled workers and allowed the 11 million immigrants illegally in the country to obtain work permits and begin a 10-year path toward a green card and, ultimately, citizenship.

“This is not the way we want to proceed. It will not solve the problem permanently,” White House communications director Jennifer Palmieri said Thursday on MSNBC.

None of those affected by Obama’s actions would have a path to citizenship, and the actions could be reversed by a new president after Obama leaves office. Moreover, officials said the eligible immigrants would not be entitled to federal benefits — including health care tax credits — under Obama’s plan.

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.

“If the reaction to this is that the Republicans are going to do everything they can to tear this apart, to make it unworkable, the big interesting question will be will our folks sign up knowing that there is this cloud hanging over it,” said Janet Murguia, president and CEO of the National Council of La Raza.

Still, Democrats battered by election losses two weeks ago welcomed Obama’s steps.

“The last two weeks haven’t been great weeks for us,” said Rep. Joe Crowley of New York, one of 18 congressional Democrats who had dinner Wednesday night with Obama. “The president is about to change that.”

Among the tools available to Republicans, who will have majorities in the House and Senate at the start of the year, are spending bills. A current spending measure to keep the government running expires next month and one option was to approve another short-term spending bill until February, when Republicans are in full control of Congress.

“We’re looking at what our levers are, and that’s clearly the funding power,” said Sen. John Cornyn of Texas, a member of the Senate Republican leadership. He added, however, “None of us want any unnecessary drama associated with the funding process.”

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