President Obama’s move Thursday to unilaterally legalize millions of illegal immigrants immediately vaults immigration reform to the forefront of the 2016 presidential campaign while diminishing prospects for cooperation between the White House and the Republican Congress that will be seated in January.

The issue could frame the debate in the opening months of what looks to be a crowded and competitive GOP presidential primary, say GOP operatives in the key, early primary states. Conservative activists are more likely to ask candidates where they stand on Obama’s “executive amnesty” — and they, in turn, are now more likely to define their campaigns according to their firm opposition and plans to roll it back should they win the White House.