Republican Response on Immigration Faces Hurdle

President Barack Obama prepared Thursday night to unveil far-reaching changes to the immigration system, while new wrinkles on Capitol Hill complicated his opponents’ plans to block his action.

Acting without Congress, Mr. Obama is expected to shield as many as five million illegal immigrants from deportation and offer them a chance for work permits, while overhauling the enforcement system, people briefed on his plan said. His action would represent the most significant changes to U.S. immigration policy in nearly three decades.

Hours before Mr. Obama was scheduled to outline his...