The upcoming visit by Indian Prime Minister Narendra Modi to Washington could mark a critical step in the success, or failure, of President Obama's climate agenda.

Whether India will stay the course in meeting its obligations under December's Paris climate change deal will be a top item on the agenda when Obama meets with Modi Tuesday.

India has not officially joined the Paris agreement, and it is crucial that it does before President Obama leaves office in January, according to proponents of the plan.

Both leaders have expressed a commitment to taking action on climate change, said Andrew Light, a former State Department adviser on India and climate change, on a Friday call with reporters. He was part of the Paris climate change negotiations.

Light, who now serves as a distinguished fellow at the environmental think tank World Resources Institute, said climate change is definitely part of both leaders' "legacies to move forward."

The big test for Obama next week will be whether Modi will announce his intent to join the climate deal before the end of the year, he said.

"The Paris text says that when 55 countries, representing 55 percent of global emissions have joined the agreement — not just signed, which is what they did in April, but depositing their instruments of ratification with the U.N. and join — then the agreement goes into force," Light said.

"The U.S., China and countries representing 50 percent of global emissions have already announced that they would [join] by the end of the year," he added.

But, thus far, 17 parties have joined, barely scratching the surface of the 55 percent of global emissions needed to make the deal stick. The 17 parties represent only 0.04 percent of global emissions, Light said.

The U.S. says it will officially join the agreement as soon as possible this year. "It's very important that we do that early. That we do this before the end of this year. That we do this before the end of this president's term," Light insisted.

But if India were to sign sooner, it would secure the deal.

"If India would join, that would put us over the hump of the 55 percent," he said on the call.

"More than the mathematics of getting us over the hump, it would send a great message both to the U.S. domestic audience and to the world that India is not only acting on climate change, but the United States and India see that, for potentially different reasons, this is the right agreement to move the world forward."

So you will see a lot of discussion between the U.S. and India on whether or not India will agree to join," Light said.

The issue is that about 25 percent of the Indian population does not have electricity, and coal is the cheapest way to get power to them. As more Indians gain access to electricity, analysts expect the country to become a top greenhouse emitter as its economy grows. And India has taken a hard line that developed countries that already are emitting the most greenhouse gases should take most of the responsibility.

Manish Bapna, the World Resources Institute's managing director, said timing is critical.

"There is a short window ... to demonstrate the commitments made [in Paris] into action on the ground," Bapna said.

He also downplayed reports that suggest India and other large developing nations were not taking their commitments seriously.

"Following Paris, there have been some concerns that some of the bigger developing countries are not actively working to implement the commitments made in the runup to Paris, and I strongly disagree," Bapna said.

He said India has made a huge commitment to increase renewable energy, even though it plans to continue to use coal. India is also levying a fee on coal to make it less attractive for electricity production, Bapna said.

Modi announced last year that he wants renewable and non-fossil energy to amount to 40 percent of India's total power supply by 2030.

But Light says that will take financing and the help of the U.S. to accomplish. He said that another key piece of the talks in Washington will be cooperation on energy financing.

Light wrote in a June 1 blog post that joint efforts between the two countries to cooperate on clean energy finance have been "painstakingly slow."

"The two leaders should push for faster, verifiable results," he wrote. "More important, government-to-government cooperation must be expanded well beyond the United States and India.

"The stakes for failure are immense. There are far less sustainable pathways for overcoming India's energy access problem than the current government's ambitious plans on renewable energy. Modi and Obama can chart a cooperative course next week toward a more sustainable future for us all."