Republican lawmakers are headed for a showdown over illegal immigration, an issue that exposes a deep and bitter rift within the GOP. The drama will unfold when Congress returns early next month and turns to finish an emergency spending bill to fund the Iraq war. The House version, approved before the Easter break, carries tough immigration restrictions, reigniting a long-simmering battle with the Senate over how to deal with the growing illegal population. It is a conflict that President Bush scarcely needs as he tries to unite his party behind contentious Social Security changes and judicial nominations. Meeting Wednesday with Mexican President Vicente Fox, Bush promised to continue pushing Congress for a program allowing temporary guest workers. That accommodation is the opposite of what House conservatives are seeking with the crackdown on asylum seekers and state driver's-license requirements for illegal immigrants that they attached to the Iraq bill. Bush acknowledged the limits of his influence: "I'm not a member of the legislative branch," he told Fox. The immigration debate pits one core GOP constituency (law-and-order conservatives) against another (business interests that rely on immigrant labor). One camp wants to tighten borders and deport people who are here illegally; the other seeks to bring illegal workers out of the shadows and acknowledge their growing economic importance.