"A federal judge stepped into the fight over Arizona's immigration law at the last minute Wednesday, blocking the heart of the measure and defusing a confrontation between police and thousands of activists that had been building for months. Coming just hours before the law was to take effect, the ruling isn't the end. It sets up a lengthy legal battle that could end up before the Supreme Court — ensuring that a law that reignited the immigration debate, inspired similar measures nationwide, created fodder for political campaigns and raised tensions with Mexico will stay in the spotlight. Protesters who gathered at the state Capitol and outside the U.S. Embassy in Mexico City cheered when they heard the news. The governor, the law's authors and anti-illegal immigration groups vowed to fight on. ""It's a temporary bump in the road,"" Gov. Jan Brewer said. The key issue before U.S. District Judge Susan Bolton in the case is as old as the nation itself: Does federal law trump state law? She indicated in her ruling that the federal government's case has a good chance at succeeding."