WASHINGTON — 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 today, using an 8 p.m. address to announce that he is sidestepping Congress and ordering his own federal action on immigration.

The reaction from congressional Republicans has been swift and fierce, heralding the start of what could be one of the most pitched partisan confrontations of Obama’s presidency.

His 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.

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 on Facebook.

An executive action that makes 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 younger than 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.