Boston — The Midwest may well be America's heartland, but as far as New England Democratic voters are concerned, their six-state region is ''Hartland.'' With a 53-to-29 percent win over Walter Mondale in the Tuesday's Connecticut primary, Gary Hart now holds nearly a 3-to-2 edge in the region's Democratic delegate seats.
Yet despite his clean sweep of the party's primaries and caucuses from Maine to Connecticut, Senator Hart may not go to next July's Democratic nominating convention with a majority of New England's 269 delegates. That's because nearly one-third of those delegates have not been chosen yet - including 48 from Massachusetts and all 17 allotted to Vermont.
That task lies in the hands of the area's Democratic Party leaders and activists. Although the recent primaries and caucuses may have some impact, most of the New England Democratic heavyweights are Mondale enthusiasts.
Mr. Mondale received the backing of all four of the area's Democratic governors - William A. O'Neill Jr. of Connecticut; Joseph E. Brennan of Maine; Michael S. Dukakis of Massachusetts; and J. Joseph Garrahy of Rhode Island. And although that apparently didn't help Mondale much among voters, it could help him fill the rest of the delegate seats.
In Vermont, for example, the March 6 primary was nothing more than a political ''beauty contest,'' and Hart's 71 to 20 percent win over Mondale may have no bearing on the outcome of the state's April caucuses or the state Democratic convention later on, where the 17 delegates to the party's San Francisco nominating convention will be picked.
Four years ago, then-President Jimmy Carter won by a landslide in the Vermont primary, but Sen. Edward M. Kennedy of Massachusetts went on to win the majority of Vermont's national convention delegates.
With the state's most prominent Democrat, US Sen. Patrick J. Leahy, on the Mondale bandwagon, Mondale's prospects for winning Vermont delegates are enhanced.
None of New England's four Democratic governors is about to desert the Mondale campaign, as disappointed as they are over their candidate's failure to carry their states' popular votes.
Of the six Hart wins in the region, the narrowest was in the March 4 Maine caucuses, where he beat Mondale 50 to 43 percent. This gives the Colorado senator a 19-to-9 delegate edge so far in that state.
Next to his Vermont landslide, Senator Hart's strongest New England showing was in Tuesday's Connecticut vote. Unlike elsewhere in the region, voters in the Nutmeg State primary had to be registered Democrats. Crossover voting was forbidden, and those registered as independents were excluded.
The Connecticut results thus may be a more accurate measure of the two contenders' comparative appeal among the state's electorate. Jesse Jackson gained 12 percent of the vote there.
In the Feb. 28 New Hampshire primary, in which the Hart candidacy gained its momentum, the senator upset Mondale 40 percent to 28 percent, but each got nine delegates.
Two weeks later, on Super Tuesday, Hart carried Massachusetts 40 to 26 percent, giving him a 33-to-22 delegate edge over Mondale so far. Hart's victory the same day in Rhode Island was 45 to 35 percent. That assured the Colorado Democrat of 12 delegates to Mondale's 10.
Of the 181 New England national convention delegate seats nailed down thus far, Hart has 96; Mondale, 68; George McGovern, who dropped out of the race, 13; Mr. Jackson, 1; and 3 uncommitted.