Judge Robert Simpson of the Commonwealth Court of Pennsylvania seems to assume that legislators have a high-minded public purpose for the laws they pass. That’s why, on Wednesday morning, he refused to grant an injunction to halt a Republican-backed voter ID law that could disenfranchise hundreds of thousands of poor and minority state residents in November.

He wrote in his ruling that requiring a government-issued photo ID card to vote “is a reasonable, nondiscriminatory, nonsevere burden when viewed in the broader context of the widespread use of photo ID in daily life,” as if voting were equivalent to buying a six-pack of beer or driving a car.

The requirement will have a disparate impact on minorities, who tend to lack government IDs at a higher rate than the general population and tend to vote Democratic. Judge Simpson acknowledged he was aware of the remark by Michael Turzai, the Pennsylvania House Republican leader, that the voter ID requirement would win the state for Mitt Romney in November. But there was no proof, he said, that other lawmakers shared that view, and, even if partisan interests were part of the motivation for the law, they are not enough to invalidate it.

There is no evidence that Judge Simpson contorted law and precedent to reach his conclusion. He even described Mr. Turzai’s comment as “disturbing” and “tendentious.” But his ruling, in a case brought by potentially disenfranchised voters, is a clear and disturbing illustration of the way Republicans have manipulated legislation for their own ends, placing a veneer of civic responsibility on a low-minded and sleazy political ploy.