The Reagan administration, to everyone's surprise, is stalling a decision on where to base the MX mobile missile in a delay that pits the Pentagon against the State Department and delights the environmentalist lobby.

The delay over whether to base this country's most vital new weapon on land or sea is fraught with potential dangers. It raises the disturbing question of whether President Reagan, who is totally committed to rapid MX deployment, is kept from abreast on whether and how his desires are carried out by competing bureaucratic power centers.

Ronald Reagan himself is partly to blame for the latest procrastination over where the United States should base the 10-warhead missiles desperately needed to give the U.S. land-based system protection against possible Soviet attack. Reagan pledged while campaigning in Nevada and Utah to take a look at Jimmy Carter's decision to base the mobile system there despite fierce environmentalist opposition.

But what should have been a pro forma Pentagon review with a foregone conclusion may be heating up into a major test. Favoring sea-based deployment, a position he espoused while deputy director of the CIA, is Deputy Defense Secretary Frank Carlucci. Defense Secretary Caspar Weinberger privately warns that environmentalist lawsuits could conceivably tie up the Carter-approved Nevada-Utah plan "for years." But Weinberger says he has an open mind on basing and a public commitment not to let the new study delay deployment of the system, expected to start in 1985.

Why, then, has Weinberger told his panel of experts they have until "June or July" to make their report? The question is particularly relevant for another reason: National security and budget officials in Reagan's White House are committed to Nevada-Utah basing. They worry that another long delay in the ever-receding "final" decision will do exactly what Weinberger privately warns against: give environmentalists that much more time to mobilize for a total assault on the Nevada-Utah plan.

Secretary of State Alexander Haig is quietly advancing a blockbuster rationale of his own against what the Pentagon calls "going to sea." If environmentalist and other political pressures are allowed to overturn the Nevada-Utah decision, Haig predicts an irreversible torrent of political reaction in Europe against modernizing NATO's land-based nuclear systems.

Boiled down, that means environmentally sensitized West Germans would physically block the nuclear modernization program agreed to by North Atlantic treaty states (NATO) in December 1979 if the United States knuckled under to political threats or legal suits by its own environmental lobby.

European statesmen visiting here have made this point hard to Haig. They reson that any U.S. decision to "go to sea" would be interpreted as a valid excuse for Europeans to demand that NATO's new nuclear weapons should also be based on boats (which military specialists say would be impossible). When the visiting Europeans warn Weinberger that moving MX to sea would create massive political problems for NATO, he not only appears to be unimpressed but at least on one occasion argued that seabasing the MX might be the best deployment in view of environmentalist delays.

Yet President Reagan has a precedent to ask Congress for a special exemption from lawsuits and other legal delaying actions now being planned by the environmentalists (by no means confined to Nevada and Utah). Congress gave the Alaska pipeline project such an exemption nearly five years ago. The project was built to specifications laid down by the Environmental Protection Agency, but it was immune from most special-interest lawsuits.

A Reagan request for similar treatment for the Nevada-Utah-based MX would get quick attention; the national security aspect is far graver in protecting America's land-based missile system than in any oil shortfall.

Moreover, White House advisers say that the courts have been friendly to Uncle Sam in rejecting environmentalist lawsuits involving military work. Federal courts have been loath to grant injunctive relief when government attorneys stake their defense on grounds of national security.

Accordingly, the preference of Carlucci and other officials for a sea-based system has little to do with environmentalists and much to do with arcane debate over weapons strategies that was resolved last year by the Pentagon after years of agonizing indecision. More indecision is not needed at this point, which is why some White House aides hope Ronald Reagan will himself end the delay forthwith.