Is there an opening for the Left in American politics? Jesse Jackson, as he made plain at last month's Rainbow Coalition conference, is betting there is. So is Machinists' Union President William Winpisinger. So is Michael Harrington, the Democratic Socialist who is sponsoring the New Directions conference this weekend at the Convention Center. Almost all the political pros in both political parties are betting the other way. They certainly have the better chance to win. But the odds are perhaps not quite as heavily in their favor as they think.

If so, that would be a change in American politics. The opening for the Left in the last 10 years has narrowed down to almost nothing. As recently as 1976, Democratic presidential candidates were expected to support a politics of income redistribution: a guaranteed national income, government jobs programs, national health insurance, progressive tax reform. But even as Democrats took control of government, they edged away from this agenda. Pat Moynihan no longer favors a guaranteed income, Tip O'Neill has quit pushing government jobs, Ted Kennedy is now not for national health insurance, Bill Bradley and Dick Gephardt as well as Dan Rostenkowksi want to lower nominal tax rates on the rich rather than raise them.

You can argue that Democrats have abandoned the politics of income redistribution because they depend heavily on big contributors or because they have been persuaded intellectually; but they have done so -- and in times when the income distribution has become more rather than less uneven. They have done so for the reason most elected politicians do most things: they have found an insufficient political demand for the political commodity they were prepared to supply.

The hopes for an opening to the Left depend on increasing that demand by enlarging the electorate. In 1984, 46 percent of those eligible did not vote. They tended to have lower incomes and were more likely to be minorities than those who voted. In some polls, at least, they say they'd be more likely to support income redistributionist policies than current voters are. If we could expand the electorate, the Left argues, we'd win.

I'm not sure it's that easy. For the past 20 years it has gotten easier and easier to register and vote -- but turnout declined steadily from 1960-62 until slight upticks in 1982-84. And nonvoters won't necessarily be Leftish Democrats. Blacks were 9-to-1 anti-Reagan in 1984, but Reagan won't be running again, and other Republicans may get more black votes. Hispanics were about 40 percent for Reagan in 1984. The largest number of newly registered voters in 1984 were evangelical Christians, who went 81 percent for Reagan. Blacks were outregistered by evangelicals in North Carolina and made the difference in Jesse Helms's victory over Jim Hunt. Deeply held religious beliefs and strongly held personal values may count for more in shaping voters' attitudes than their income levels.

Nor is there a permanent American proletariat large enough to vote the Left into office. For years Left politicians have pointed to unchanging income distributions as evidence of lack of opportunity. But real incomes have risen over the years, and the University of Michigan "longitudinal" study of individuals over three decades shows that most Americans move up and down the income ladder over their lifetimes. It may not seem sensible to soak the rich even for low-income voters if they believe reasonably that these voters will become comparatively rich themselves sometime soon.

So where is the opening for the Left? Among, I think, those whose high hopes have been most cruelly dashed. This is what happened in the 1930s. The auto and steel industries -- growth industries then and previously hostile to unions -- were organized by CIO unions, which became the institutional heart of an income- redistributionist Left. New York City, a boom town in the 1920s, became a Left stronghold 20 years later.

Something like that could happen again, at least in a severe economic downturn. Jesse Jackson has been ridiculed for advocating antique notions ("parity") of aid to farmers and for calling for higher gas prices to help energy workers. But it may make sense for him to appeal to those farmers and Oil Patch residents whose hopes have been so cruelly dashed by the latest turns in the economy. Intellectuals who yearn for a politics of income redistribution may be disturbed because Jackson's policy prescriptions seem demagogic. But there is no guarantee that a politics of income redistribution will be intellectually respectable or that a politics that takes some people's money and gives it to others has to be respectful of civil liberties or tolerant of cultural diversity. The 1930s could have given us Huey Long rather than Franklin Roosevelt and Europe some very nasty dictators. The crash of the 1980s, if there is one -- and if there isn't the new American Left is unlikely to win except here and there, no matter how many voter registration drives it sponsors -- could bring a politics that many people sympathetic to the Left will not find entirely palatable.