Funny how quickly some principles collapse when given the right kind of shove. One day, the Republican Party is rock-ribbed restrictionist, dedicated to the proposition that unauthorized immigrants are an invading army of job stealers, welfare moochers and criminals whose only acceptable destiny is to be caught and deported — the border fence forever, “amnesty” never. The next day: never mind. The party suddenly discovers the merits of a working immigration system. Senators like John McCain and Lindsey Graham, who once bravely supported bipartisan reform but slunk away late in the last Bush administration, are scratching at the door again, as if the last five years never happened.

All it took was an election in which millions of Latino voters — many of them the wives and husbands, sons, daughters, grandchildren, cousins, co-workers and friends of those despised “illegals” — overwhelmingly chose President Obama over the man who promised to be deporter in chief. They rejected Mitt Romney by 3 to 1, according to exit polls. Asian-Americans did, too. Republicans looked at a changing America, saw a future of decline and irrelevance for the party, and concluded that immigrants weren’t so bad after all.

This poses an opportunity and a challenge for Mr. Obama, who promised to tackle immigration reform in his first term and did not, and is firmly on the hook to do so now. He says he will push reform early, and he looks well positioned to get something done. His allies in Congress need to step up and help.

There is a long way to go before an immigration deal is struck, but the president and Congress can draw its outlines clearly, starting now. Any worthwhile reform must give 11 million undocumented immigrants a way to live within the law as American citizens. Mr. Obama’s stopgap move to protect young immigrants and students from deportation by a regulation similar to the Dream Act was sensible and necessary. But these Americans-in-fact deserve the chance to be Americans on paper, too. So do their parents, and whoever else in the 11 million wishes to journey from “them” to “us.” Republicans are floating schemes for temporary legal status for workers without a clear path to citizenship. Mr. Obama should make clear that basic equality demands more than that.