Immediately after the busted rescue mission in Iran, I asked half a dozen senior Democrats about the possibility of an inner-party revolt that would force an open convention and the unhorsing of President Carter. All scoffed at the notion. One -- the man who figured to have the keenest feel for delegate sentiment and the most to lose from a Republican victory -- said: "I dwell in the world of reality."

Reality changes, to be sure. But my mini-sampling suggests that several hugely unlikely conditions will all have to come together in a brief period before a serious threat to Jimmy Carter can be mounted in the Democratic Party. The conditions for a Democratic earthquake are as follows:

First, Ronald Reagan has to open a commanding lead over Carter in the opinion polls. At present, senior Democrats do not take the former California governor all that seriously. They believe he is shallow, inexperienced and prone to make bad mistakes on the stump. They think that with attention going first to Kennedy, then to George Bush, then to John Anderson and now to Carter, Reagan has yet to be exposed to the full glare of media scrutiny. They expect he'll sink, not rise, in the polls.

Nor are they impressed by the pulling power of the Republican Party. I cited an analysis of the Wisconsin and Kansas primaries made by Horace Busby, a former aide to President Johnson who now works as a business consultant. The Busby analysis shows that 60 percent of the eligible voters participated in the Republican primaries as against only 40 percent in the Democratic primaries.

Those numbers suggest, to me and to others, the possibility of a Republican landslide in the fall. But the senior Democrats seem confident that with a little work they can continue to hold comfortable majorities in both houses. So one condition necessary to shake them into action is convincing evidence that Reagan can beat Carter so badly as to threaten Democratic control of Congress.

A second condition is a strong showing by John Anderson in the polls, in raising money and in getting his name on the presidential ballot. The senior Democrats all think of a two-man race -- Carter v. Reagan. They acknowledge widespread disaffection in the Democratic Party. But they place the unhappiness chiefly among liberal Democrats on the two coasts. They believe that in the end, the liberals, faced with the prospect of Reagan, will return to the fold and vote for Carter.

Anderson is important because he represents respectable middle ground. If he begins to show strength, the liberals will not feel pinned to a choice between Reagan and Carter. They can deliver a message by going to Anderson in droves. In that event, nobody could predict the outcome, and the confidence of the senior Democrats in Carter would again be badly shaken.

A third condition is the withdrawal of Sen. Edward Kennedy. The senior Democrats, in touch with both the Kennedy and the Carter camps, report an unprecedented degree of personal animosity. The two leaders have contempt for each other and, as usual, the loyalists down the line feel even more bitter.

The Kennedy forces think of Carter as a Democrat who has adopted Republican policies -- a renegade. The senator and his people talk of carrying the fight through the primaries and onto the convention floor. Even if beaten there, it is not clear the Kennedyites will back Carter for president.

The Carterites regard Kennedy as a would-be usurper. They are prepared -- as a recent session of the committee making arrangements for seats and floor passes at the convention in New York showed -- to play hardball all the way. While it is possible that Kennedy would withdraw to make room for a third candidate, Carter would never get out unless Kennedy quit first.

The supreme testing time for all these possibilities remains weeks off. A Kennedy withdrawal is not even thinkable until after the June 3 primaries in California, Ohio and New Jersey. The true dimensions of Gov. Reagan's appeal will probably not emerge until he finally nails down the nomination -- presumable in the California primary. As to the Anderson campaign, it is only into its first days. The effort to put the candidate's name on the ballot in 50 states, for example, is, while going strong, still in the study phase.

Big changes in outlook are equally unlikely after the Republican convention in mid-July. The first nomination tends to freeze developments until after the second.

So there is only a short season -- five weeks after June 3 -- for a Democratic earthquake. Given the number of things that have to happen, and the reluctance of movers and shakers to make them happen, the odds, even in this year of the roller coaster, overwhelmingly favor the nomination of Jimmy Carter on the first ballot.