BY A COMFORTABLE margin, the residents of Arizona voted for Proposition 200 this week, a ballot measure designed to slow the flow of illegal immigrants into their state. If it isn't struck down in court, the law will oblige federal clerks to check the citizenship documents of people registering to vote or applying for state and local benefits. In addition, the law requires public employees to report illegal immigrants to the federal authorities or face misdemeanor charges.

While intended to prevent the distribution of welfare, Proposition 200 leaves open the question of what other "public benefits" might be included. Do public hospitals count? Vaccination clinics? During the campaign, opponents of the measure said the law could be interpreted to mean that a state employee working in a public park could be charged for failing to turn in illegal immigrants having a picnic. Advocates pointed out, on the other hand, that in the absence of federal enforcement of the law, state enforcement is at least preferable to the Arizona vigilante groups that have sprung up along the Mexican border.

Particularly if it really does force state employees to demand citizenship documents from everyone they encounter, the law could well persuade immigrants, legal and illegal, to leave Arizona for other states, spreading not only the problem but the frustration that led Arizonans to back Proposition 200 since it was first proposed.

But illegal immigration is not a problem that can be sensibly dealt with state by state: The federal government controls the country's borders. President Bush signaled his interest in immigration reform a year ago, announcing a plan to introduce a form of temporary work visa, but he dropped the plan almost immediately after encountering opposition in his own party. Some in Congress, including most of the Arizona congressional delegation, have supported other kinds of temporary visas, but none were deemed politically viable before the election. Now that the election is over, it is time to end this particular state revolution before it starts -- by solving the problem in Washington.