Miami Beach — A big Mondale banner is pasted on the outside of the building over the entrance to Sid Rubin's tiny ground-floor office here, just a few blocks from the ocean. Inside, Mr. Rubin sits behind a small, dusty, metal desk, wearing one Mondale button on his sport shirt and another on his sweater.
In a few words, this veteran of local politics sums up what appears to be part of Walter Mondales's problem with Gary Hart.
''He (Mondale) represents the old-line philosophy of the Democratic Party . . . Roosevelt . . . Kennedy.''
Mondale represents, he says, ''the old urban politics of the Democratic Party.''
That's just the message that Gary Hart, now with a New Hampshire and a Maine win to this credit, has been trying to get across.
But Rubin, a 30-year resident here, and secretary of this neighborhood's Democratic committee, is still banking on old-style ward politics and help from labor to carry this area for Mondale. The local Democratic committee selects a candidate to endorse, he explains, ''and they (voters) fall into line.''
Florida has a record of being unpredictable in national elections.
Florida continues to receive a massive influx of new residents, most of whom arrive with little knowledge of Florida's political past. Some come with former allegiance to the Republican Party, some are Democrats, others have no strong political ties, making the forecasting of winners in this state a tricky business.
In the last six presidential elections, for example, the state has gone Republican three times and Democratic three times. But this kind of volatility extends back even further.
V. O. Key Jr., in his book ''Southern Politics,'' published in the late '40s, titles his chapter on Florida politics ''Every man for himself.'' And no one seems to have much coattail effect here. Even candidates for governor have often avoided endorsement of the encumbent, in order not to be later tied to an outgoing administration.
During national elections in Florida, ''the Democratic Party has always dissolved into factions,'' says Christopher L. Warren, an assistant professor of politics at Florida International University.
The state is still conservative enough that former Florida Gov. Reubin Askew, who dropped out of the presidential race after poor showings in Iowa and New Hampshire, was seen nationally as one of the most conservative candidates, but in Florida is seen as ''quite progressive,'' Mr. Warrensays.
Mr. Askew had been counting on solid support from his home state - if he made it that far in the race. But even if he had, his organization here, according to a number of political activists in Miami, was not very strong.
Neither is Gary Hart's organization.
Sid Rubin, while still maintaining confidence that Walter Mondale can win in Florida, is having some second thoughts.
''This is the old-line Democratic Party that we have on the line,'' he said by telephone after Hart's win in Maine. ''Out of the blue comes a new candidate that arrived (on the national political scene) during the Vietnam war. (His) values are new.
''It starts us to wonder what are our political values. We thought we had this thing all sewed up.'' But, he added, ''We're still optimistic.''
Rubin, an accountant, pauses during his interview in his work/campaign office to quote a $7 fee for helping a young man fill out a short tax form. He knows this area well, he says. It is the low-income section of the otherwise more affluent Miami Beach.
Part of the larger congressional district he lives in extends through the Cuban-American Little Havana section of Miami and through a black neighborhood. In the Miami Beach portion there are a high number of low-income elderly voters, many of them quite liberal Democrats, he says.
He admits that it appears that ''the younger Democratic voter does look toward Hart for new ideas.'' But he also says the ''bread-and-butter ideas'' that are important in his area are not new: social security, medicare, welfare. He says the AFL-CIO is helping the Mondale campaign here by distributing literature to its members.
Rubin opens an old, four-drawer, metal filing cabinet in a corner and pulls out some crumpled voter tally sheets of his own, showing an overwhelming preference for Democrats in the 1982 congressional elections.
''This is not Reagan territory,'' he says.
Is it Mondale territory? Yes, he still insists. But Gary Hart's wins in New Hampshire and Maine did surprise him.