The G.O.P. platform approved Tuesday in Florida included tough language on many expected issues like abortion, but also takes a stand on an issue that has historically been out of the party’s mainstream: Agenda 21.

“We strongly reject the U.N. Agenda 21 as erosive of American sovereignty, and we oppose any form of U.N. Global Tax,” the platform reads.

Agenda 21 is a 1992 United Nations resolution that encourages sustainable development globally. Although it is nonbinding and has no force of law in the United States, it has increasingly become a point of passionate concern to a circle of Republican activists who argue that the resolution is part of a United Nations plot to deny Americans their property rights.

In a New York Times article in February written with my colleague, Kate Zernike, we reported about activists aligned with the Tea Party disrupting local city and state land use planning meetings nationwide to denounce sustainable efforts to reduce energy use — including bike lanes on public streets and smart meters on home appliances. The activists see such community projects as the first steps in a plan to limit individual rights.

Since that article, land use officials in many parts of the country have complained of anti-Agenda 21 disruptions, most recently in Notasulga, Ala.

Most of those pushing the Agenda 21 theory have been largely on the margins of their own party. But the inclusion of language for Agenda 21 in the Republican Party platform could mark a turning point, said Tom Madrecki, a spokesman for Smart Growth America, an advocacy group that works to limit sprawl.

“Though the actual language of the platform does not say anything besides ‘we oppose Agenda 21,’ the fact that it’s in the platform gives credence to something that just shouldn’t get any,” he said.

He said he’s concerned that formalizing opposition to Agenda 21 will bring more disruption and will “continue halting beneficial conversations about community planning.”

Mitt Romney’s campaign did not return e-mails asking whether the nominee supports the inclusion of formal opposition to Agenda 21 in the platform.