On the thorniest of political issues, President Obama has embraced the enforcement-first position on immigration that he criticized during last year's presidential campaign, and he now says he can't move forward with the type of comprehensive bill he wants until voters are convinced that the borders can be enforced. Having already backed off his pledge to have an immigration bill this year, Mr. Obama boosted his commitment to enforcement in the budget released Thursday. The spending blueprint calls for extra money to build an employee-verification system and to pay for more personnel and equipment to patrol the border.