One thing that makes the Republican nomination contest a gripping psychodrama is that the two main antagonists have diametrically opposed -- and matching -- problems.

Vice President Bush cannot be mean enough. Every time he lands one on Congress or the news media, he bows to a crowd that used to think he was a sissy and now sees him as an honor graduate in assertiveness training.

Sen. Robert J. Dole (Kan.), on the other hand, cannot be mean at all. He is dark and sleek and resembles a Doberman pinscher. The trouble is that he used to act like one, and his most celebrated turn at nastiness is on videotape. In the 1976 campaign, in a debate with vice presidential nominee Walter F. Mondale, he was bitter and self-destructive, like the Nixon of the falsely labeled 1962 "farewell" news conference. Dole, a disabled decorated veteran of World War II, snarled about "Democrat wars" and scared voters. He's now minority leader of the Senate and a changed man, patient, almost mellow.

Bush is now moving about slaying dragons, the "distortions" and "innuendoes" of the news media, "hopeless" congressional leadership. Dole has no villains he could plausibly beat up on. For him to attack the media would be like kicking his dog: Reporters gather around his desk every morning he is in town and trade jests. How can he trash Congress, his home for 27 years?

Critics nag him about vision. They say he is a legislative mechanic, focused on what can be done, not what should be done. He draws Democrats, but succession-minded Republicans are unswayed.

Bush is the man ahead of him in the line. Given the hierarchical structure of the GOP, their feeling that any change in orderly procedure can bring bad luck, he has to provide them a reason why Bush should be bumped. The fact that Dole draws Democrats has no appeal for succession-minded Republicans.

Bush may seem to some a collection of anxieties bound together by an almost hysterical ambition for public office. But Republicans feel if the crown prince wears a clean shirt and shows up for work every day, they should not tamper with protocol. They sense that President Reagan favors Bush.

Just looking at the dirt-poor farm-boy from Kansas and the eastern preppie, voters know they are very different.

But Dole cannot define the greatest difference between them. He cannot use the most powerful weapon at hand, Bush's conduct in the Iran-contra crisis and afterwards. Bush has been frantically inconsistent. He started with "Hey, I had nothing to do with this" and has, with some rude shoves from the media, shifted into the gear of "Ask me anything, I dare you."

Dole at once grasped the enormity of the crisis and stepped forth, alone among Republicans, to demand an investigation. In their first shock that an adored president had been slipping arms to Iran, the GOP applauded. But he can't brag about it now.

Having survived the engulfing embarrassments of the summer hearings, Republicans have drifted back to doting on Reagan. They bristle at the mention of Iran, tune out reminders that the vice president was involved. "It was Reagan's scandal, not Bush's," wails a Bush loyalist. Even in Iowa, where farmers are aggrieved, the president's approval rating is 60 percent. Dole is expected to win the curious ritual of the Iowa caucuses -- his rivals say handily, his handlers say barely -- but he is 18 points behind in New Hampshire, where on Feb. 15, the first secret ballots of 1988 will be marked.

Says Tom Rath, a seasoned New Hampshire strategist and a senior adviser to Dole, "If you attack Bush on Iran-contra, you may strike Reagan, and 80 percent of New Hampshire approves of the president."

Dole's campaign chairman, William E. Brock, former Republican national chairman, minds, of course, that Dole cannot exploit his advantage, which is that a majority of Americans believe that Bush has not come clean. But Brock vetoed an Iran-contra television ad for New Hampshire. "Conservative Republicans think it's disloyal to bring it up. It's family," he said.

So Dole has to mumble around the edges. He cannot ask Bush whether his Iran-contra deportment is a measure of how he would handle things in the Oval Office. He cannot ask him about Donald Gregg, his national security adviser, who was told not only about the secret contra weapons airlift but about the corruption in the process -- and never breathed a word to Bush. Gregg did not regard it as "vice presidential information."

Those who know Dole best know with what relish he would ask Bush what "vice presidential information" is -- and what "presidential information" would be. But he can't. For an expert needler, it is perhaps not hell, but surely purgatory. And he can't see that holding his sharp tongue is getting him anywhere, either.