The Supreme Court ruled today that the Trump administration could deploy a national database, using sensitive data including citizenship information and Social Security numbers, to help states hunt for ineligible voters.

The decision is a win for President Trump, who has been pressing his administration to prove that noncitizens are voting illegally, despite evidence that it is very rare.

However, states are unlikely to remove large numbers of voters off their rolls before November’s midterms because federal law mandates a 90-day “quiet period” before elections.

The court’s three liberal justices dissented from today’s decision, with Justice K...