A REPUBLICAN PRESIDENT is on the verge of negotiating an arms control treaty and is being subjected to withering criticism from presidential candidates -- of his own party. That's what's been happening since the administration announced progress in the talks between Secretary of State Shultz and Soviet Foreign Minister Shevardnadze.

Only one of the six Republican candidates has been standing foursquare behind Mr. Reagan on this one, and that -- no surprise -- is Vice President George Bush. Under the circumstances he would be acting irresponsibly to do otherwise. There's no reason, anyway, to doubt that Mr. Bush does favor the administration's stand, which seems on balance the wisest view. Mr. Bush owes the voters more information on where he'd take arms control from here. But he is right to wait until after the president fords this particular stream.

Another Republican candidate, Bob Dole, carefully limits his questions to issues of verification. He is concerned about treaty language and about how a treaty would address past as well as prospective Soviet arms control violations. These are sensible questions for one who, as Senate Republican leader, will play a major role in ratification. If those questions are settled, Mr. Dole thinks the treaty is one step in the right direction.

The other four Republicans voice serious doubts. Each is concerned about the Soviet advantage in conventional forces in Europe. Jack Kemp is also worried that Senate Democrats will insist on their interpretation of the ABM treaty and thus foil progress on SDI. Pete du Pont is also concerned about verification and worries that the new treaty would open a "window of gullibility." Alexander Haig thinks a treaty should do more to lessen the Soviet edge in land-based missiles and regrets the lack of linkage to "aggressive Soviet behavior in Afghanistan and elsewhere." Pat Robertson wants our European allies to beef up their conventional forces and wants U.S. negotiators to demand that the Soviets reduce their conventional forces.

There's a risk that, taken together, the candidates' attempts to be more Reaganist than Mr. Reagan will undercut the president's position by suggesting his party is not behind him. The question is whether the candidates in truth so dislike the deal Mr. Reagan is working toward that they are willing to put it in jeopardy -- and with it the political gain that is widely anticipated for the president and his party if the treaty comes off.