President Obama played a major role closing the deal at last year's COP21 negotiations in Paris. He has taken steps to ensure that Republicans won't be able to easily scuttle the agreement. He knows that Republicans will fight against efforts to reign-in climate change.
To get around the need for formal ratification by the Senate he successfully pushed to change the Paris deal from a treaty to an agreement. maneuvers
He has also accelerated the timelines so that it will be even more difficult to withdraw from the deal, even if a Republican were to win the 2016 presidential election.
By signing the agreement and accelerating the timeline, it will be that much more difficult for a the next president to withdraw. As quoted in the Washington Post, Daniel Bodansky, a scholar of international environmental law at Arizona State University and a former attorney at the State Department focused on climate change said:
"(T)he next president could not withdraw until sometime in 2019, and the withdrawal would not be effective until sometime in 2020."It will be difficult for the US to withdraw even if one of the current slate of three remaining Republican nominees becomes president. Considerable international pressure would be brought to bear making such a withdrawal no easy diplomatic feat.
There is also a provision in Article 28 of the agreement that states that any nation wishing to withdraw must wait four years after the deal enters into force.
However, there is only so much President Obama can do to protect the agreement. While a Republican administration may not be able to kill the deal outright, they would almost certainly fail to comply.