President Obama on Wednesday will declare a national monument in southern New Mexico, delivering a win for environmentalists but angering ranchers and local law enforcement, who say the land restrictions will end up creating a safe haven for drug cartels to operate within the U.S.

Mr. Obama will declare about 500,000 acres as the Organ Mountains-Desert Peaks National Monument. About half of that land is expected to be set aside as wilderness, meaning it will be closed to vehicles and construction.

Local ranchers say it’s a land grab that will interfere with their grazing rights, and border security advocates said the move will make it tougher for federal agents and local police to patrol the land, leaving a security gap that Mexican smuggling cartels will exploit.

“This is about opposing so many thousands of acres that is going to create nothing more than a pathway for criminals to get into this country to do their criminal acts,” Dona Ana County Sheriff Todd Garrison told The Washington Times in a telephone interview Monday....