The Democrats will win, but

Democrats will gain several governorships, hold their own in the United States Senate, and pick up anywhere from 20 to 35 congressional seats in elections next month.

Party spokesmen and leaders will proclaim these gains as certain signs of even greater victories, and a retaking of the White House, in 1984. They will be talking through their hats.

Democratic gains next month will have little to do with public confidence in the party's political philosophy or in its capacity to govern. They will be due to normal off-year electoral trends and to the sad state of the economy.

In the aftermath of next month's victories, there will be two perils for the Democrats and for the country.

The first peril is that the party's leaders and candidates will believe that simple opposition to Reagonomics will be a sufficent substantive and political platform for the next two years. One of the Democrats' two principal presidential aspirants said as much in an early-October news interview.

The second, related peril is that the same leaders and candidates will lose interest in the policy rethinking which began within the party in the wake of Republican victories in 1980.

''Why upset anybody?'' they will reason. ''Don't offend the pressure groups and key constituencies by examining their sacred cows; let Reagan and the Republicans bury themselves.'' In other words, politics as usual.

That kind of reaction would be inexcusable - and it will not be excused by the voters in 1984.

The domestic and international challenges, especially the economic challenges , facing the country will not be met by blind continuation of Old New Deal approaches by the Democrats any more than they have been by President Reagan's nostalgic and disastrous return to Daddy Warbucks economics and ideology.

Over the past 18 months there has been a slow but unmistakable consensus forming among congressional Democrats (it already exists in large part among many state and local officeholders) around a new agenda. That agenda, partially reflected in the House Democratic Caucus's recent report on long-term economic policy and in bits and pieces of proposals by various senators, congressmen and presidential candidates, centers on the restoration of US international economic competitiveness; improvement of productivity; stimulation of new investment in reseach and development; rebuilding of public infrastructure; revitalization of key old industries and encouragement of promising new ones; upgrading of both physical and human capital; design of new and cooperative institutional arrangements among business, labor and government; and, finally, development of a Democratic version of New Federalism which will enhance governmental efficiency and responsiveness without abdicating government's 50-year role as guardian of economic and social justice.

The agenda, you say, would seem self-evident. But all its aspects involve painful shifts and tradeoffs for those with a stake in the status quo. They imply changes in economic policy, tax law, entitlement programs, and ways of doing the public business which will anger the disciplined single-issue and special-interest pressure groups with votes and money which are accustomed to Winning Through Intimidation.

Both major parties' national platforms have for the most part since 1968 been compilations of the ''wish lists'' of their most demanding and insistent factions. The legislative process has been hostage to the same syndrome. The American people have recognized as much. That is why many of them do not vote - or vote for only one or two candidates on a ballot.

Stark economic reality should force Democrats, even in the wake of next month's apparent victories, to face down the factions and pressure groups and face up to the need for new policies and approaches which meet the larger, national interest. The voters are not fools. They see that reality all too clearly in their daily lives.

For the first time since the 1930s, serious and basic questions are being asked about American society and others in the industrialized West: Are we truly capable of governing ourselves? Does capitalism have a future? Are both rich and poor nations destined to share a crisis brought on by their own selfishness? Where are the leaders large enough for the times?

Americans are better off than others, yet even here we seem unable to achieve any constructive unity or sense of larger purpose or the ability to address the obvious.

Given all of this, a Democratic Party which seeks to regain power and then to govern through smart one-liners about Reaganomics will and should be dramatically rejected by the citizens it seeks to lead.

Those Democratic candidates who succeed next month shouldn't spend much time in self-congratulation. Rather they should be grateful that, this time, the political bullet hit the Republican in the next foxhole.