The Supreme Court allowed the Trump administration to deploy a system that merges personal data from across the government. Questions remain about its reliability. In advance of the upcoming midterm elections, the Trump administration has embarked on an intensive campaign to prove that noncitizens are voting illegally, despite evidence that it is very rare.

The Supreme Court’s order on Friday allowing the Trump administration to merge sensitive personal data stored across federal agencies gives the government a wide-ranging tool in its effort to identify noncitizens and ineligible voters. The decision moves President Trump closer to his longstanding goal of creating a national citizenship database. To find such voters, the administration has harnessed a Homeland Security Department system called the Systematic Alien Verification for Entitlements (SAVE) tool. Last year, the administration vastly increased the scope of the D.H.S. system by giving it access to data from the Social Security Administration.

It remains up to state and local election officials if they want to use the SAVE tool, meaning it is most likely to be deployed in Republican states. The Supreme Court ruling does not compel its use. The case underscores a long-running debate between immigration restrictionists and privacy and voting rights activists. George Fishman, a former senior D.H.S. official, said information about citizenship had been scattered across various federal agencies. Ben Berwick, an election law attorney at Protect Democracy, said the database in its current form was unlikely to give states a strong justification for clearing their voter rolls.

Multiple organizations including the League of Women Voters and the Electronic Privacy Information Center challenged the database on privacy grounds.