The Massachusetts Democrat said Thursday in a statement India is trying to pressure the United States into bending on its anti-proliferation laws.

"India must understand that the legal boundaries established by the Congress for nuclear cooperation cannot be overstepped during negotiations between the two Executives," Markey wrote.

The two countries' leaders have been negotiating the deal since 2005. It would provide India access to U.S. nuclear technology and material, something banned by U.S. law since India has nuclear weapons and hasn't signed key international treaties. The deal would make an exception.

There are two main sticking points: India has contested certain provisions that would hamper what it calls a sovereign right to develop its nuclear weapons program, and the U.S. Congress wants New Delhi to break development ties with Iran.

India wants to increase nuclear energy as a way to meet rising energy demand.

It has maintained a relationship with current Bush arch nemesis Iran, forging ahead on deals that include investing in natural gas supplies.

"India is not only getting chummier with Iran, but it looks like New Delhi is actively courting Tehran," Markey said. "Their growing relationship is making a lot of folks in Congress increasingly skeptical about the long-term viability of this nuclear deal."