Campaign's decisive dozen.

Discouraged Democrats describe the 1984 presidential election this way: Walter Mondale is like a man wrestling with an elephant. He pulls and punches, but the elephant hardly notices. Mr. Mondale's only chance now is for the elephant to step on a banana peel.

As the campaign races toward November, Democratic strategists are relying more and more on the ''banana peel'' theory. President Reagan looks so strong that unless he slips and makes some major error, even Democrats see little hope for Mondale.

Mondale aides, just like everyone else, watch the polls. The national surveys by Gallup, Harris, and others are discouraging. But the state polls, in such pivotal battlegrounds as Illinois and Texas, are even more disheartening to Mondale supporters.

One method of analyzing the current election is to look at the ''decisive dozen,'' 12 states that are large enough together to elect the next president. They range from California (23.7 million people, 47 electoral votes) at the high end to Indiana (5.5 million people, 12 electoral votes) at the low end. Together these 12 states have 279 electoral votes, or 9 more than are needed for victory.

Mr. Reagan leads in all 12. In fact, Democratic and Republican insiders agree that the President's margin is growing, even in big states where Mondale a month ago was once getting very, very close.

Illinois is a vivid example. A month ago the race there had tightened up, according to private polls. It looked as if traditional big-city Democratic power in the Chicago area, combined with farmer unhappiness in downstate Illinois, could combine to give Mondale a stunning upset in the state.

But no longer. Private polls now put Reagan's lead in Illinois at more than 10 points. (Some polls are just above 10, but at least one is reported nearer to 20.)

Then there are the examples of New York State, Pennsylvania, and Massachusetts - all considered part of the traditional Democratic base.

Even Reagan enthusiasts concede that the President will probably have a tough time carrying Massachusetts on Nov. 6. In New York, Geraldine Ferraro's name on the ticket should almost ensure Mondale of a winning margin. In Pennsylvania, anger in steel country and prolonged aftershocks from the 1981-82 recession have given the Democrats powerful issues.

Yet in all three states, Reagan leads, the polls show.

A Reagan aide notes: ''A year ago we were in trouble in Pennsylvania. Yet now we're in pretty good shape - ahead more than 10 points at the moment, we believe.''

A Democratic insider counters that ''Mondale's best shot'' seems to be in Pennsylvania, New York, and Massachusetts. But he admits that Mondale currently trails by ''10 to 15 points'' in that area of the country.

Then there's California, which Mondale has made a prime target. The Mondale emphasis on California has puzzled some experts. After all, it's the President's home state. He's never lost an election there. Yet Mondale goes back again and again, and his aides say he is ''confident'' of victory.

The polls may explain why Mondale is focusing so heavily on California, despite the views of skeptics. If he is to have any chance of beating Reagan, Mondale must cut into Reagan's strength in the Sunbelt. That means he must whip the President in at least one of the big-three Sunbelt states - California, Texas, or Florida.

New Republican surveys put Reagan ahead by 30 points in Florida, about 25 points in Texas, but ''only'' 17 points in California. Although California is clearly uphill, it could be Mondale's best Sunbelt hope.

Although time is clearly running out for Mondale and the Democrats, insiders in both parties agree that the race is almost certain to be much closer when voting day comes.

Tradition plays a role in this. Florida and Texas, for example, are states in which Democrats usually begin to ''come home'' to the Democratic Party as election day nears.

If Mondale were within 5 to 10 points of Reagan in those states at the moment , he would be in strong contention. It is only the huge spread that is leading some Democrats to conclude that his chances in the Sunbelt, as one put it, are ''slim and none.''

Briefly, here's an overview of the outlook in the ''Big 12,'' according to insiders in both campaigns.

New York State. A must for Mondale. New York is as important to Democrats as California is to Republicans. Mondale should be ahead in New York by now. The fact that he is not shows how serious his problems are.

California. All Reagan.

Texas. Mondale did well here against Gary Hart in the caucuses. Democrats had great hope that voter-registration efforts could put the state into their column. That hope seems to be waning.

Pennsylvania. One of Mondale's best prospects. Economic hard times still linger. His strategists hope he can remind Pennsylvanians why they are Democrats.

Illinois. Mondale has done a fade-out. It shapes up as a major Mondale disappointment. But he was once close there, and that gives Democrats hope. Probably a ''must'' state if Mondale is to win.

Ohio. Mondale got close there for a while. Now Reagan has more than a 10 -point cushion.

Florida. Reagan country.

Michigan. This once seemed ripe for Mondale. The scars of recession still show. But a tax-revolt mood appears to be helping Reagan.

New Jersey. All Reagan.

North Carolina. A tough Senate race is the main attraction on Nov. 6. But even if Republican Sen. Jesse A. Helms is ousted by his Democratic foe, Gov. James B. Hunt Jr., it will probably go with Reagan.

Massachusetts. Perhaps Mondale's strongest state. If he loses Massachusetts, it's over.

Indiana. Reagan again.Chart-

%Reagan Carter Bush Mondale Northeast 47.6 41.8 Connecticut 48.2 38.5 Maine 45.6 42.3 Massachusetts 41.9 41.7 New Hampshire 57.7 28.4 New Jersey 52.0 38.6 New York 46.7 44.0 Pennsylvania 49.6 42.5 Rhode Island 37.2 47.7 Vermont 44.4 38.4 North Central 50.7 41.2 Illinois 49.6 41.7 Indiana 56.0 37.7 Iowa 51.3 38.6 Kansas 57.9 33.3 Michigan 49.0 42.5 Minnesota 42.6 46.5 Missouri 51.2 44.3 Nebraska 65.9 26.0 Ohio 51.5 40.9 Wisconsin 47.9 43.2 South 50.8 44.8 Alabama 48.8 47.5 Arkansas 48.1 47.5 Delaware 47.2 44.8 D.C. 13.4 74.8 Florida 55.5 38.5 Georgia 41.0 55.8 Kentucky 49.1 47.6 Louisiana 51.2 45.7 Maryland 44.2 47.1 Mississippi 49.4 48.1 North Carolina 49.3 47.2 Oklahoma 60.5 35.0 South Carolina 49.4 48.1 Tennessee 48.7 48.4 Texas 55.3 41.4 Virginia 53.0 40.3 West Virginia 45.3 49.8 West 54.3 34.2 Alaska 5 4.3 26.4 Arizona 60.6 28.2 California 52.7 35.9 Colorado 55.1 31.1 Hawaii 42.9 44.8 Idaho 66.5 25.2 Montana 56.8 32.4 Nevada 62.5 26.9 New Mexico 54.9 36.7 North Dakota 64.2 26.3 Oregon 48.3 38.7 South Dakota 60.5 31.7 Utah 72.8 20.6 Washington 49.7 37.3 Wyoming 62.6 28.0 Totals 50.7 41.0