With six months to go before the real battle begins between Republicans and Democrats, the Republican candidates are already caught up in what George Bush might call a doo-doo debate over the vice president's role in the Iran-contra affair.

Before it's over, Bush will almost certainly have to clarify his working relations with the president -- if he wants to trade on vice presidential experience. He may also have to straighten out some nagging questions about exactly what sort of hand he did have in the biggest national security fiasco of the Reagan presidency.

When the joint congressional investigation was unable in its November report to find the vice president's fingerprints on the policy making, the Bush camp was mighty relieved. Then, a week or so ago, congressional investigators belatedly fished a memo out of a White House computer reporting that the ''VP'' was ''solid in taking the position that we have to try'' the arms-for-hostages initiative.

The memo was signed by Vice Adm. John Poindexter, the former national security adviser who had assured the committee that he ''had not withheld any information . . . that I can accurately recall.'' The memo was dated Feb. 1, 1986, two weeks after President Reagan had signed off on the arms-sale decision. Bush had already publicly acknowledged that he had supported the president's decision once it had been made -- while privately expressing ''reservations.''

But never mind. The Poindexter memo suddenly became ''the first evidence . . . concerning the vice president's position,'' according to a committee statement. And so the fun begins.

Alexander Haig jumped in with a statement saying, ''The American people have a right to know what George Bush advised the president.'' Sen. Robert Dole, the vice president's main challenger, took the same tack, citing the ''new memo'' as evidence that Bush ''supported arms to the ayatollah.''

Come now. There is no public ''right to know'' what advice a president gets in confidence from his vice president. Still less did the Poindexter memo suggest anybody's support for ''arms to the ayatollah.'' The whole point of the transaction, however misguided, was to strike up a relationship with supposedly ''moderate'' Iranians at odds with the ayatollah.

But Dole had it half right when he said that it would be ''in Bush's interest'' to be a bit more precise about his role. The more you examine the record laid out in the congressional report, the more evident it is that the position that the vice president has been taking is highly vulnerable to the sort of scrutiny it is going to continue to get.

In his campaign autobiography, ''Looking Forward,'' Bush claims he was ''deliberately excluded from key meetings.'' If he had known that Secretary of State Shultz and former secretary of defense Weinberger ''had serious doubts,'' he says he would have called for a meeting of the National Security Council. The president might then have seen the whole project ''as a gamble doomed to fail.''

Yet the congressional report says Bush attended at least one ''full NSC meeting'' on Jan. 7, 1986, in which Shultz and Weinberger ''continued to object strenuously'' to the whole project.

There doesn't seem to be any ready explanation for the vice president's inability to remember his presence not only at this meeting but at others recorded by the congressional investigators when the pros and cons of the arms-for-hostage scheme were discussed. The veep's aides do insist there is nothing significant about the absence of any record of his views: it has been his practice -- as it is the president's -- not to tip his hand in large meetings. Privately, one-on-one with the president, the vice president often comes down on one side or another of an argument and freely expresses dissent, they insist.

That's something Bush vows he is ''not ever going to discuss.'' But the vice president has already broken that rule once. In his testimony to the Tower commission, he revealed his ''reservations.'' He expressed concern, the commission reported, ''about what he perceived as the extent to which the interests of the United States were in the grip of the Israelis'' in the arms transactions.

If that concern wasn't deep enough to push Bush into outright opposition, together with Weinberger and Shultz, he ought to say so -- clearly. The record won't sustain the case that he wasn't sitting in when the rights and wrongs of the arms-for-hostage deal were being argued out.