Evan Vucci, Associated Press
President Barack Obama speaks in the East Room of the White House in Washington, Thursday, Nov. 20, 2014, during a ceremony to award the National Medals of Science, and the National Medals of Technology and Innovation. The awards are the highest honor bestowed by the United States Government upon scientists, engineers, and inventors.

WASHINGTON — President Barack Obama is poised to claim broad authority to grant work permits to millions of immigrants living illegally in the United States and to protect them from deportation. But Republicans are vowing an all-out fight against it.

"Congress will act," Senate Minority Leader Mitch McConnell warned on the Senate floor Thursday, hours before Obama's 8 p.m. EST address.

Obama "will come to regret" his action, McConnell said. "We're considering a variety of options. But make no mistake. When the newly elected representatives of the people take their seats, they will act."

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 at least 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 that's expected to be expanded. Changes also are expected to law enforcement programs and business visas.

However, the plan would leave the fate of millions more unresolved. With more than 11 million immigrants living in the country illegally, Obama's actions would not offer specific protections to more than half.

Still, Obama was expected to ensure that many of those not covered -- immigrants who have lived illegally in the U.S. for 10 years or more or parents of citizens or permanent residents who have been in the country fewer than five years -- would be given a lower priority for deportation, essentially sanctioning what is already current practice.

"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.

On Thursday, Obama discussed the need for an overhaul of the immigration system in the context of science and technology, saying the U.S. benefits from innovations and discoveries by scientists and researchers who come here to pursue their work.

"Part of staying competitive in a global economy is making sure we have an immigration system that doesn't send away talent but attracts it," Obama said at a White House ceremony recognizing achievements in science, technology and innovation. "So that's what I'll be talking about a little bit tonight."

But the vehement reactions of Republicans, who will have control of Congress come January, made clear that Obama was courting a serious partisan confrontation.

Some on the right pushed for using must-pass spending legislation to try to stop Obama's effort. One lawmaker — Republican Rep. Mo Brooks of Alabama — raised the specter of impeachment.

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 other 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.

And 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 green cards 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 direct path to citizenship, and his actions could be reversed by a new president after he 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 the executive orders.

"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."

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