WASHINGTON – Massive demonstrations nationwide have helped propel legislation to legalize millions of undocumented immigrants. But immigration experts and critics caution that the legislation, if enacted, could generate a legacy of unintended consequences. Senate legislation would double legal immigration, legalize millions currently in the country illegally and open the border every year to hundreds of thousands of “guest workers.” The Senate debated the legislation in March and April but failed to complete action before going on a recess. The House approved enforcement-only legislation in December that would make unlawful presence in the country a felony, boost efforts to stop illegal...