EVERGLADES NATIONAL PARK, Fla. — President Obama on Wednesday made his first visit while in office to the Florida Everglades, choosing the backyard of a former Republican governor of the state, Jeb Bush, and its Republican senator, Marco Rubio, to demand action on climate change in a critical battleground in the 2016 presidential election.

Standing in front of a marsh at the entrance of the 1.5-million-acre Everglades National Park, Mr. Obama commemorated the 45th Earth Day, designating a national historic landmark at the Miami home of the environmentalist Marjory Stoneman Douglas, who led efforts to rescue the vast area of grasses and forests.

“You can see what makes this unique landscape so magical,” Mr. Obama told a small crowd of community leaders and Park Service employees and guests. But he added a warning: “Climate change is threatening this treasure and the communities that depend on it, which includes almost all of South Florida. And if we don’t act, there may not be an Everglades as we know it.

The message was an effort to reinforce what the president hopes will be a central part of his legacy: a sustained push to get the United States and the rest of the world to confront the effects of climate change.