There is a legitimate and important issue behind the challenge to George Bush to disclose his advice to President Reagan on the Iran arms sale decision. But it is a dangerous issue for Bob Dole to raise, because he may be equally vulnerable to the underlying question.

The real issue for both men is whether they have the courage of their own convictions. It is an issue for them as the Republican presidential nomination battle approaches its first tests next month, because Ronald Reagan has established a model, at least in the minds of Republican voters, of a president who acts on the basis of his beliefs. Arms-to-the-Ayatollah notwithstanding, Reagan is seen by most Republicans (and even some Democrats) as a man who has stood up for what he believes.

In the contest for Reagan's inheritance, it is the trailing candidates who most clearly display that attractive and important Reagan trait. Pete du Pont and Jack Kemp, though far back in the polls, better reflect Reagan's readiness to adopt "radical" policy alternatives and sell them to an initially skeptical public. Along with Al Haig, they have not hesitated to say plainly where they disagree with existing policies -- even when it has put them at odds with Reagan. And Pat Robertson has displayed far more than Bush or Dole his mastery of Reagan's patented technique for turning away angry criticism with soft answers and ready smiles -- without trimming on policy.

These four men have been unable to exploit their "conviction politics" so far, because of other doubts about their leadership capacity. Their credentials -- as former governor of Delaware, a member of the House, Richard Nixon's last chief of staff and a former television preacher -- do not get them over the threshold of presidential credibility in their first try for the office.

Bush and Dole have passed that threshold, in part because they ran for president once before and because they already hold responsible positions as vice president and Senate Republican leader. Each is now attempting to win by suggesting that the other lacks the vital ingredient of leadership embodied in the notion of principle and conviction.

Those doubts are inherent in Bush's career. It raises questions about his ability to stand up for himself and his beliefs in critical situations. Bush has been notably unsuccessful in gaining the elective offices he sought, failing twice for the Senate and once for the presidential nomination. But he has risen to higher and higher appointive posts by pleasing, flattering and serving men in power.

Such a career breeds caution, and Bush, from all the available evidence, was notably reticent when the crucial decisions were being made on arms sales to Iran. At first he pleaded ignorance, but the unfolding record shows he was present -- and apparently largely silent -- at meetings where Secretary of State George Shultz and Secretary of Defense Caspar Weinberger expressed their strong opposition to the president's policy.

It looks like a clear-cut case of Bush's deference undercutting the critical judgment he might have applied as a former CIA director and as the administration's designated specialist on anti-terrorism policy.

Bush has not put that suspicion to rest. But he has been clever by demonstrating an almost aggressive readiness to challenge his critics on the issue. In every one of the debates so far, he has swung hard at the rivals or the journalists who pressed him on the question. He and his advisers have grasped the point that it's hard to prove a guy lacks backbone when he's telling you to put up your dukes.

Dole, on the other hand, has yet to demonstrate a clear instinct on how to handle his "conviction politics" problem. It, too, is rooted in his career. For 27 years he has been in Congress, for all but six years in the minority. He has mastered the art of the half-loaf, of trading advantageously with those who hold high cards, for whatever he could extract in policy or political terms. It's a necessary role for a congressional leader, but it rarely allows him to display his principles.

On many of the vital issues of the last seven years, Dole plainly has "risen above principle" to accommodate Reagan -- the very sin of which he accuses Bush. As chairman of the Senate Finance Committee, he passed the deep tax cuts Reagan wanted in 1981 -- against his better judgment -- and has spent the past six years trying to repair the resulting drain of federal revenues.

In last week's Des Moines Register debate, Dole himself offered another example of his trimming. Asked about his 1986 vote to sustain Reagan's veto of the sanctions against South Africa -- a veto the Republican-controlled Senate overrode in protest of the apartheid policy -- Dole said: "If I were president I might have a little different view, but as Republican leader, I saw no need to pass it and embarrass the president."

If the leadership issue between Bush and Dole is a matter of who stands up to the president on a matter of principle in foreign policy, Republican voters may have a hard time making up their minds.