Mulford told Karan Thapar on "The Devil's Advocate" programme to be aired on CNN-IBN on Sunday morning.
"It certainly would not be revived and offered by any administration, Democratic or Republican, before the year 2010, which is after the life of this administration in India," the envoy said in yet another effort to stir the powers-that-be in New Delhi to finish the deal, which will end over three decades of India's global nuclear isolation.
When asked if he was saying it is "now or may be never", he replied: "That's pretty close to it."
India has yet to negotiate a safeguards agreement with the International Atomic Energy Agency (IAEA), a key step towards operationalising the nuclear deal, even after four rounds to negotiations. The Indian government can, however, proceed with the nuclear deal only after its Left allies approve the draft of the IAEA pact.
It's only after India concludes a pact with the IAEA that the Nuclear Suppliers Group (NSG) can consider the India-US nuclear deal and decide on changing its guidelines to allow the resumption of global civil nuclear commerce with India.
Mulford's matter-of-fact reminder about time slipping away for the nuclear deal comes amid an increasingly grim outlook about the future of the deal.
Mulford, however, spoke candidly when asked whether the failure to implement the deal will affect India-US ties. Although Mulford said it will not damage the relationship, he admitted that it will be a setback to years of efforts to overcome "some of the distrust, suspicions and misunderstandings" that dogged ties between the two sides.
"...and the civil nuclear deal was supposed to be the vehicle that would lay those things to rest forever. I think there has to be some concern about elements of trust and discretion at the core of that relationship," he said.
Calling the Indo-US nuclear deal "India's passport to the world," the envoy rebutted statements by India's communist parties - the CPI and the CPM - that the US wanted to convert India into a subordinate ally in South Asia.
"That's completely untrue", he said. "The US wants to assist India in achieving its global vision of emerging as a major economic power in the world," he added.
The envoy also sought to allay anxieties in India about the impact of the 123 agreement on India's right to test a nuclear device.
"Not at all. It can make that decision at any time. It's a sovereign state... it's very clear that India is free to do as it wishes with regard to future testing," he said.
However, when the envoy was questioned about the implications of the American right of return on any strategic reserves of fuel that India may build up, in the event of the termination of the 123 pact, he assured that it would not undermine India's strategic reserves.
"If that strategic reserve is created in the right way, which is India's business to do, and managed in the right way, which again is India's business to do, there should be absolutely no problem in maintaining the sanctity of that reserve," he said.
"We've agreed that the right of return may apply to our fuel but we have limited the application of that to fuel from other sources," he added.
Mulford also admitted that although obtaining an unconditional clearance from the NSG was "not going to be easy", it was "achievable".