The main push behind this is to stop uranium mining north of the Grand Canyon. Not in the Grand Canyon where uranium deposits are eroding into the Colorado River all the time, but north of the Canyon where the uranium deposits are hundreds to thousands of feet below the surface. The footprint of the uranium mines that exist now is small, generally more than 20 acres, consisting of a mine hoist, some buildings, and the facilities for trucks to haul the ore to Blanding, Utah. The people pushing for this want the president to use the Antiquities Act to create the national monument by Executive Order. Congress will have no say whatsoever in the creation of the national monument. The monument could be managed by the BLM, the Forest Service, by both, or by the Park Service. All the other so-called threats are just smoke and mirrors. It's the uranium mining that gives the enviros the heartburn. Their idea of green energy, or getting away from coal generated electricity is for us regular non-rich and non-famous people to just stop using electric power. The uranium deposits with the highest concentration of uranium, and the easiest to mine safely in the USA are those between the Grand Canyon park boundary and the Utah state line. The "documented problems" with uranium mining that the website mentions date back 50 or more years.
Sending letters to congress people is not likely to matter. Look for this national monument, and at least three more, to be created within the next 18 months. What we want is less than nothing.