Well-intentioned U.S. policies sometimes work out in absurd ways, but this is hard to top: In a few weeks, India, the world's largest democracy, will probably elect as its next prime minister a politician who for nearly a decade has been prohibited from setting foot on U.S. soil.

The banned Indian official is Narendra Modi, a longtime Hindu nationalist who is the prime ministerial candidate of the opposition Bharatiya Janata Party, or BJP. Nine years ago, U.S. officials denied...