Bay State still a Mondale stronghold?

If Walter Mondale is to rebound from his setbacks in New Hampshire primary and Maine, a first-place finish in the March 13 Massachusetts primary will be crucial.

This is how some veteran political observers, including a few of Mr. Mondale's Bay State supporters, view what appeared to be one of the candidate's major strongholds just a few weeks ago.

Now Mondale activists here are all the more determined to win next Tuesday's primary by defeating Gary Hart of Colorado.

The outcome may depend on how the three other Democratic contenders - John Glenn of Ohio, Jesse Jackson, and George McGovern of South Dakota - do in the Bay State primary.

Mondale forces can take little comfort from a Boston Globe poll, published Monday, which showed Senator Hart with a 12-point lead over the former vice-president (41 percent to 29 percent). By contrast, a similar preference sampling in mid-January showed voters favored Mondale 43 to 3 percent over Hart.

The latest poll, completed the day before Sunday's Maine caucuses, placed Senator Glenn a distant third with 12 percent, down from 19 percent six weeks ago. Messrs. Jackson and McGovern each scored 4 percent.

Mondale boosters here, although displeased with the way their candidate fared in New Hampshire and Maine, question whether the Hart momentum can continue. They are counting on organized labor and other groups to turn out in large numbers for the Democratic primary.

Unionists and others sympathetic to Mondale were highly visible at at Sunday's Massachusetts Democratic State Committee fund-raiser. Of the party's five remaining White House hopefuls, all but Glenn spoke at the function.

By far, Mondale received the loudest and most enthusistic ovation. Some in the audience responded with boos when Senator Hart sniped at Mondale, saying he lost the Maine caucuses despite being better organized, better financed, and backed by the AFL-CIO.

In Maine, the endorsements of the state's prominent Democrats (including Gov. Joseph E. Brennan, former Gov. Kenneth M. Curtis, and US Sen. George Mitchell) apparently did not help Mondale. In Massachusetts, too, he has more than a few Democratic Party heavyweights in his political corner.

Mondale's showing on the March 13 ballot is partly a test of the influence within voter ranks of Gov. Michael S. Dukakis, US House Speaker Thomas P. O'Neill Jr., and Boston Mayor Raymond L. Flynn.

Senator Hart, on the other hand, must rely largely on a grass-roots effort, comprising few established party activists.

Glenn's Massachusetts aides, including his state chairman Donald Davenport, vehemently deny their candidate is all but writing off the commonwealth. Since his third-place finish in New Hampshire, Glenn has been focusing his efforts in the South. He has been scheduled to spend much of Thursday in the Bay State, but such plans may be scrubbed so he can campaign in the South. The Ohio senator is continuing to rely heavily on radio and television spots to push his Massachusetts campaign.

After their Sunday appearances, at which each reaffirmed strong advocacy for nuclear arms control, candidates Hart, Mondale, Jackson, and McGovern went their separate ways on the Bay State campaign trail Monday.

McGovern, who carried only Massachusetts and the District of Columbia in the 1972 presidential election, is continuing to devote full time in pursuit of votes in the Bay State primary. If he doesn't finish first or second in next Tuesday's balloting, the former South Dakota senator says he will quit the presidential race.

Crucial to the outcome on Tuesday are the supporters of the three candidates who dropped out after the New Hampshire primary. Several prominent Massachusetts backers of California Sen. Alan Cranson have joined the Mondale team.

In Maine, where Cranston's forces appeared to be second only to Mondale's, much of the Cranston support is believed to have swung to Hart after Cranston quit the race.

''It certainly helped, especially in some of the towns,'' says Richard J. Mainan, a political science professor at the University of South Maine. Professor Mainan, a Portland resident, also attributes Hart's victory to the momentum he gained in the New Hampshire primary.