The Iowa caucuses on Monday not only mark the beginning of the 1980 presidential campaign but are also, as a visit to that state suggests, apt to foreshadow the end.

Three candidates with big reputations -- Ronald Reagan, John Connally and Edward Kennedy -- have been campaigning in ways and from motives that make them distinctly vulnerable. Two -- George Bush and Jimmy Carter -- have positioned themselves for a quick kill.

Reagan seems pushed to seek the Republican nomination mainly to satisfy supporters who saw him come so close in 1976. In keeping with the front-runner strategy laid out by his campaign manager, John Sears, he took and held his distance from the rest of the pack. As long as voters were merely asserting vague preferences, he held a long lead in the polls.

Failure to appear with other candidates, however, fed doubts that he had something -- maybe age, maybe lack of competence -- to hide. His standing in the polls dipped shortly -- a sign of soft support. He might lose in the caucuses, or gain only a narrow victory. In each case, the unraveling of the whole Reagan effort would proceed apace.

Connally seems driven to seek the presidency by the desire for vindication over the suspicious that survived his acquittal in a bribery trial. He deliberately asserts presidential positions on policy matters. Especially in the board rooms of those supposed to command money, power and respect, he has waged a coast-to-coast campaign.

But the run up to the nominating convention is a string of local elections. So Connally comes on as a stunted giant -- a Shakespearean actor in a play by Neil Simon. A poor showing in Iowa could well be the token of an insurmountable problem.

George Bush, in contrast, has come to Iowa the way the voters like to see a candidate -- hat in hand as an aspirant. He has visited the state often and established an effective organization.

The polls show him rising steadily and, when I toured with him the other day, his campaign had about it the smell of victory. A win or a good second would send Bush into high gear. For he is well organized and not only in New Hampshire, to go all the way.

Howard Baker, by a stong showing, could, of course, dim the Bush star. A burst of last-minute appearances in the state has combined with support from the popular governor, Robert Ray, to advance his cause. If Baker draws strength from Bush, the Republican race becomes a free-form scramble.

The Senate minority leader seems to secure in his niche, however, to hunger for the presidency. His campaign got off to a slow start, and his organization is still weak. So the chances of his derailing Bush seem small.

On the Democratic side, fate figures more than appetite as the driving force behind Edward Kennedy. As Carter stumbled over the summer months, volunteers pushed Kennedy in ways hard to disavow. When he assented in September, he looked unbeatable.

But he had not transformed his Senate posture into a presidential perspective. He entered the race without articulating new themes, and was thus driven back by the force of events to the far-out liberal corner. If he does even reasonably well in Iowa on Monday, it will be thanks to powerful organization.

For Carter has moved strongly to exploit the opening. He inherited from 1976 a good standing in Iowa. The White House put together a vigorous organization, Still, there remained a weakness of performance.

By fixing attention on the hostages in Iran, however, Carter first insulated himself against attack. Just as failure to spring the hostages started to become evident, attention switched to Afghanistan. Confrontation with the Russians there made what was essentially a retreat look like an advance. Now those who assail the president seem unpatriotic.

Freezing events in mid-passage does not come easily. Further deterioration of the American position abroad could cause the Carter stock to sink again. The campaign would then turn into a long, slow war of attrition with incalculable results for both parties. But if the president can hold the present stance that long, he is in positon to dispatch Kennedy by mid-March.