Maryland's top Democrats, trying a last-minute campaign blitz to return the state to the Democratic column, see the Washington suburbs as the key to a victory for Walter F. Mondale.

Democratic strategists say that Prince George's and Montgomery counties are critical areas in the presidential race because the traditional Democratic backbone of Baltimore city is now partially offset by increasing numbers of Republican voters in Baltimore County. In 1980, the Democrats won Prince George's County and Baltimore city, but lost Montgomery and Baltimore counties.

Yesterday two Maryland Democrats, Sen. Paul S. Sarbanes and Rep. Steny H. Hoyer of the 5th District, leading the latest campaign "zip trip" by statewide officials, spent about 90 minutes handing out literature supporting Mondale and his running mate, Rep. Geraldine Ferraro (D-N.Y.), to morning rush-hour commuters at the New Carrollton Metro station. The response was clearly positive.

"I saw Vice President Bush yesterday and it took all my self control not to say something nasty to him," said one woman, as she stuck a Mondale-Ferraro button on her shirt.

But despite the affirmation of support from Prince George's County voters at the subway, a senior citizens' home and a rally in Upper Marlboro, Mondale's chances of carrying the traditionally Democratic county -- and the state -- are far from certain. He has been trailing in the state, according to recent polls. Even with its preponderance of federal employes and minorities who oppose many of Reagan's policies, Mondale is not assured a victory in Prince George's.

"If we don't win the 5th congressional district, which includes most of Prince George's County , he is going to lose badly throughout the country," said Hoyer who, paradoxically, is viewed as a shoo-in for reelection to Congress for his third term. "But the fact that the question is being asked whether Mondale will carry the county tells you something."

Michael Frazer, Mondale's state campaign director, said yesterday that new voters who supported Jesse L. Jackson's candidacy in the state's presidential primary will enable Mondale to win Prince George's with more than the 16,000-vote margin the Democratic ticket had there in 1980.

The "zip trip" was perfected in 1982, when Gov. Harry R. Hughes, Attorney General Stephen H. Sachs, Sarbanes, and state comptroller Louis Goldstein were all up for reelection. As they did yesterday in Prince George's, several of the state's best-known Democrats campaigned together to reinforce the political weight behind the ticket.

Sarbanes, one of the first members of Congress to endorse Mondale last year, said there is an "enormous gap" between Reagan's promises and policies. He cited Reagan's appearance in Baltimore earlier this month, in which he expressed support for federal funding to dredge the harbor. The next day, as Congress prepared to vote on the continuing budget resolution, Reagan threatened to veto the measure if money for the harbor project was included.

"Reagan will stop at nothing to fool people," Sarbanes told county employes. "Are the American people going to see through this? That's the critical question."