President Bush had hoped to leave behind a nuclear cooperation deal with India that would be a legacy foreign policy and energy achievement. But for reasons beyond Bush administration control, the deal is in danger of unraveling.

When the agreement was announced in 2005, the administration called it “perhaps the single most important initiative” in 60 years of U.S.-Indian relations. Those relations have generally been chilly, although they’ve warmed greatly over the past decade. But Monday Indian Prime Minister Manmohan Singh informed Bush that “certain difficulties” had arisen with this latest milestone.

If the deal to trade in nuclear reactors, technology and fuel goes through, it would reverse longstanding U.S. policy of not engaging in such trade with countries that have not signed, as India has not, the Nuclear Non-Proliferation Treaty.

On the other hand, assuming India can be trusted not to divert the benefits of the trade to nuclear weapons, it would be a lucrative deal for the U.S. nuclear industry and a significant step in Bush’s plan to restore nuclear power as a major source of energy.

But time is not on the president’s side. Congress, where considerable skepticism must be overcome, must start work on the treaty early next year if it is to be completed while Bush is still in office.

If Bush faces some resistance, Singh apparently faces even more, maybe too much. India’s insular leftist parties, with a long tradition of dislike and distrust for the United States and its deregulated capitalist ways, have threatened to pull out of Singh’s ruling coalition, perhaps bringing down his government.

Some think Singh should take the chance but the prime minister apparently prefers a more cautious approach, perhaps waiting until after his own parliament debates the pact some time next year, again a little too late for Bush to take credit.

Over the weekend, Singh said, “What we have done with the U.S., it is an honorable deal, it is good for India and it is good for the world. But we are in the realm of politics . . . ”

So there it is, Mr. President. To paraphrase “The Godfather,” It’s nothing personal. It’s just politics.