Iowa's Democrats have sent the nation three messages about Election '84: * Walter Mondale has moved so far ahead of his seven opponents that he could lock up his party's presidential nomination by mid-March.
* Gary Hart is emerging as the leading alternative candidate, especially among Democrats looking for a ''new face.''
* John Glenn's campaign, once expected to challenge Mr. Mondale right through the spring, has fallen into deep, deep trouble.
The Mondale victory in Iowa had been predicted by just about everyone. But few had expected it would be this big, this impressive. He outpolled the second-place finisher, Senator Hart, by about 3 to 1. He whipped Senator Glenn, supposedly his nearest rival, by 9 to 1. His combined vote was nearly equal to every one of the seven other candidates combined.
Mondale showed strength across the board. Data from an ABC-TV computer study which was made available to the Monitor showed that Mondale drew 46 percent of the vote in the state's largest cities. That was to be expected, since voters there are traditionally more liberal, more pro-labor, more activist - the groups known to be most solidly in the Mondale camp.
But the same ABC study found Mondale even stronger, with 50 percent of the vote, in the state's smallest communities, with fewer than 2,500 residents. And among farmers, he claimed 43 percent support.
His weakest showing - an area that other candidates may now try to exploit - was in middle-class suburbs, where his vote dropped to 34 percent.
This broad Mondale strength has suddenly turned the Democratic campaign into what looks like a one-man race.
A Democratic Party activist who has withheld his support from Mondale so far expressed amazement at how well things have gone for the former vice-president:
''I don't think Mondale's staff could have planned things this well if they had tried,'' he said. ''Look at what has happened: John Glenn has suddenly, unexpectedly faded. Who knows why? But it has happened too late for anyone else to take his place, too late for anyone else to mount a major campaign to challenge Mondale in the South, in the big Northern states, and so forth. It looks as if he can almost coast into the nomination from here.''