GEORGE BUSH HAS set for himself a difficult task: convincing the nation that he is at once the most engaged vice president in modern times and innocent of knowledge in the Iran-contra affair. We have long found his claim difficult to believe. Last week our doubts increased.

According to media reports, Bush is the only presidential candidate of either party to refuse to take part in Marvin Kalb's excellent series of nationally televised interviews, "Candidates '88". Kalb, director of the Kennedy School's Center for Press, Politics and Public Policy, is an incisive questioner whose televised exchanges with Michael Dukakis, born-again candidate Gary Hart and Paul Simon are among the most substantial of the campaign. Kalb certainly would have questioned the vice president about evidence--including a recently surfaced memo signed by former National Security Council chief John Poindexter--that suggests Bush's support for the plan to trade arms for hostages held in Iran was stronger than he has acknowledged.

Bush's campaign maintains that he decided against participating in the "Candidates '88" interview series because of scheduling pressures. Barbara Pardue, Bush's press secretary, points out that the candidate has been interviewed by David Frost and Barbara Walters and appeared on NBC's Meet the Press. Yet the "half day" Pardue said it would take Bush to "do a good job" on Kalb's hour-long program, which is taped in the Kennedy School's Arco Forum and broadcast on PBS, shouldn't be difficult to find in the schedule of a candidate who all but resides in neighboring New Hampshire. And Bush's television interviews to date have either preceded disclosures about the Iran-contra affair or been puff jobs during which the vice president has ducked tough questions.

That's why Bob Dole's recent, forceful questioning of Bush's Iran-contra role is entirely in order. We hope that Bush recognizes the validity of concerns about his knowledge of the fiasco and soon discloses his involvement. It is not a hopeful sign, however, that so far his only concrete response to his chief rival for the Republican nomination has been to suggest--apparently without merit--a conflict of interest associated with the handling of Elizabeth Dole's blind trust and to call for the Kansas senator to release income taxes for the past 10 years. Perhaps Dole is no longer the poor Dust Bowl boy he once was, but we'd like to know whether the vice president supported the arms-for-hostages swap just the same.

With $23 million raised and a still comfortable lead in the polls, Bush, like his boss, evidently feels no need to answer tough questions. That in effect is what he told Kalb when he declined to be interviewed. What he told the American people is that he would be their president without putting his case before them.