As we approach the last week of the campaign, Ronald Reagan has just a little over 50 percent of the vote in several national polls. That should mean that Walter Mondale has a chance for a breakthrough. But Mondale has a problem: he isn't leading, so far as the polls disclose, in a single state.

The fact that the Mondale base is still missing after the candidate's two credible debate performances spotlights not only a temporary problem for this campaign, but a permanent problem for Democratic presidential candidates. A party with no base has to scurry pretty furiously to win, with very little margin for error. So it's important to understand what happened to the Democratic bases of the past.

The Solid South. That's been missing a long time: the last time the South voted solidly for a Democrat was 1944. Democrats lost their near-monopoly on the white southern vote when they supported civil rights in the 1960s; for a while they lost the white southern vote altogether. Jimmy Carter got a lot of it back, but not a majority, not even in 1976.

Race is no longer the issue that keeps white southerners voting Republican in presidential contests. Economics does: southern whites are increasingly affluent, upwardly mobile -- and out of sympathy with high tax and government programs for the poor. White southerners also have attitudes on cultural issues (traditional, not liberation- minded) and foreign policy (hawkish, not dovish) that predispose them to vote against national Democrats. Mondale will get some white southern votes, but there's no prospect for a Solid Democratic South.

The Industrial Belt. The decaying of the Democratic coalition in the industrial belt from New York through New Jersey, Pennsylvania, Ohio, Illinois and Missouri is an old story now. Ethnic minorities have long since moved socially upward and politically to the right; there are fewer union members and blue-collar workers; no one gets his Christmas turkey or job from a political machine any longer. There aren't enough people left in big cities to deliver the "traditional" majorities registered most recently in 1960 and 1964. These states were among those most closely contested by Franklin Roosevelt and the Republicans in the 1940s, but they're not going to be a safe Democratic base any time soon.

The McGovern 8. The eight states where George McGovern ran best in 1972 run along the Northern Tier and West Coast -- Massachusetts, Rhode Island, Michigan, Wisconsin, Minnesota, South Dakota, Oregon and California. None was a traditional post-Civil War Democratic state; there was no overlap with the Solid South or Industrial Belt. High-income, well-educated, high-tech states, dovish on foreign policy and liberal on economics, they looked like candidates for a new Democratic base. Mondale has been running close to Reagan in all but South Dakota, but still seems behind in each.

One reason is that economic issues have turned against the Democrats there. Massachusetts, Michigan, Minnesota, Wisconsin and California all had tax-cut movements -- and saw their local economies improve as a result. (Ironically, the big spenders there were Republican governors in the early 1970s, Sargent in Massachusetts, Milliken in Michigan and -- yes -- Reagan in California.) Tax cuts, state and federal, have in turn stimulated the high-tech industries that are so visible, especially in California and Massachusetts.

Highly educated electorates a dozen years ago saw the Vietnam War and environmental depredation as threats to their future, and looked to government to tame them. The highly educated electorates of today increasingly see high tech and computers as the promise of the future, and look to private capital formation to provide them. A Democratic Party bent on subsidizing auto and steel workers' jobs is a threat to this future; a Republican Party bent on leaving capital in the hands of investors is what they want -- and they're willing to overlook the rantings of the Rev. Jerry Falwell if that is what is needed to get it.

Mondale's most recent ad attacking the Reagan "Star Wars" nuclear defense illustrates the Democrats' problems. You see a picture of the Earth from space and then whirring computers and blinking lights, with an ominous voice warning of the danger of war. An unspoken assumption of this ad is that high-tech machinery is scary and ominous. But to whom?

Republican strategists believe that Reagan's argument in Kansas City that this new technology can prevent nuclear destruction may prove plausible to many of those voters who see the computer as their friend. A dozen years ago among educated Americans new technology seemed frightening: it brought death in war and destruction of the environment. Now, in a time of peace and a nation that has cleaned up its air and water, new technology seems more friendly and promising.

So in the final weeks of the compaign, neither the McGovern 8 nor the Industrial Belt nor the South provides a comfortable base for Walter Mondale -- a solid bloc of electoral votes to balance those Ronald Reagan has in the Rocky Mountains, the Great Plains and the fastest-growing states in the South (Florida, Texas, Virginia). The Democrats' failure to maintain their former bases in the South and the Industrial Belt is neither surprising nor alarming; issues of Reconstruction and the New Deal have long since been resolved. More ominous for them is their failure to nail down some time ago the McGovern 8, the part of the country where a candidate like Mondale might expect to run strongest; their failure to capture the imagination of the educated people who, in contrast to those in their places a dozen years ago, believe that science and technology and free enterprise and America are good things.