PAUL TSONGAS, 39, took away Edward Brooke's Massachusetts Senate seat two years ago, after having served two terms in the House. As a representative, he regularly got dismal ratings from conservative and business groups; this year's Almanic of American Politics says he "can be expected to vote for liberal Democrats on just about every issue." He appeared to be starting his Senate career as a keep-the-faith Democratic liberal -- a common breed until a week ago.

Last June, Tsongas made a surprising speech to the national convention of the Americans for Democratic Action, the sine qua non of liberal groups -- surprising because he called for new business tax credits, development of nuclear power, increased gasoline taxes and, generally, "a new liberalism." He warned that if it doesn't change substantially, "liberalism will decline."

The speech seemed somewhat precient at the time, and extremely prescient in the wake of last Tuesday's massacre of several of Tsongas' fellow Senate liberals. The election obviously raises the question of whether liberalism has a future in American politics. Post staff writer Nicholas Lemann interviewed Tsongas about that question; an edited transcript follows.

Q. Was this an ideological election? Do you think that this was a real conservative mandate as opposed to an anti-Carter mandate?

A. Well, if this was not a conservative mandate, I don't know what is. To the extent that you've ever had one in this century, I think this clearly was it. It was not only losing the White House but a virtual demolition in regard to the liberals defeated by conservatives but moderates were defeated by conservatives. And so I think the country clearly wanted a change.

Q.When people vote for a conservative, what are they voting for? What is a conservative? What makes people vote conservative?

A. My own view, which is perhaps a minority view, is tat people generally are disposed towards the Democratic mindset. I think the Democrats are perceived as being a bit more, if you will, human. But what happened was the Democrats, in terms of approach, were still living off the New Deal. And eventually, since that began to run counter to the realities of the 1970s and '80s, New Dealism bumped heads with the realities that were out there. And there was a perception the Democrats had run out of steam. And a reaching for something else. I don't think that the equivalent Republican dogma is any more attractive or workable. But it has not been tried for 25 years. There was only one other commodity on the market and that's what the voters went to.

Q. I was struck during the conventions by two things in that regard. One, the Democrats had always been the party of a sort sweeping optimism that Reagan totally took for himself. The Republicans had been rhetorically a party of caution, and Carter was now talking that way. Second, although you say the Democrats or liberals are more human, Reagan seemed much more human from his acceptance speech onward in that his basic message was, "You, the average guy, will have a better life if I'm elected." Now that's easy to say if you're not president, but Carter never came near saying that.

A. That's true, but that doesn't explain why the Senate changed. If there had been a shift of four or five senators, then I think that thesis would hold. But it doesn't explain the wholesale destruction of the liberal majority, or at least the operative majority in the Senate. There's something much deeper, more basic.

Q. Well what is it?

A. I think that there are certain movements, sweeps, that take place. And if you get caught up in those things there's not much you can do about it. And I think what happened was that for years the Republicans have had the advantage of espousing what I would consider very simplistic notions. And since they were never in a position to introduce them and get them passed, these notions were attractive.

Q. Notions such as what?

A. The notion that you could increase defense spending, balance the budget and cut taxes massively at the same time. That you could reject the SALT treaty and show the Russians a thing or two. Those kinds of statements, which are great crowd pleasers. The trouble with those ideas is that you may someday find yourself, God forbid, in power. Which is exactly where they find themselves today.

And so if you try to solve the energy crisis by unleashing the oil companies and you're dealing with a finite diminishing resource, you've got problems. And if you think that you can in essence browbeat the Soviets into a postion of acquiesence to your superiority, you've got another set of problems. If you think you can solve inflation by massive increases in defense spending, and a nonresolution of the energy crisis, you've got a third set of problems. And so what happens in essence is that all these sort of "bumper sticker economics" approaches have now come on the stage, and while the simplicity of their appeal is what made them attractive, it also makes them unworkable.

It was always assumed that you would have a conservative Reagan showing moderate tendencies because of some of the people around him, and being further moderated by the Congress. And now you have just the opposite. You have a Congress that, at least in the Senate, potentially can be even more hardline than he. And not having that filter, Reagan in many respects has the best of both worlds, and also the worst. Because he now has the power to do what he wants. If things don't work, he cannot blame it on a Democratic Congress.

Q. One other thing I was struck by in the election is that Reagan seemed to get about the same margins among people who weren't doing very well -- and I exclude minorities from this -- as he did among people who were doing very well indeed. He won the depressed industrial areas, and he won the Sun Belt. Why do you have people with such different lots in life coming down on the same side? What is it about conservatism that's appealing to both of them?

A. I see it as far more a rejection of what we offered, we the Democrats; as it is an embrace of what the Republicans had to offer. You know, the old coalition that FDR put together was basically a coalition of have-nots.

Q. Now you have the have-nots voting with the haves.

A. Now you have the have-nots in this society speaking in the traditional FDR sense, being someone other than an unemployed steelworker who may be employed a month from now. The have-nots in the society are basically minorities. So you have a shrinking base of people who viscerally would be attracted to the Democratic Party.

Q. But why is it that in 1932 the unemployed white person was attracted to government as the answer to his problems and is not in 1980?

A. In 1932 he was throwing out the existing government. That's exactly what he did in 1980, because he knew that the way things are going is not in his favor. In 1932, you had Herbert Hoover, who believed in a balanced budget when the country was going through recession, and by pursuing the balanced budget exacerbated the recession into a depression. In 1980, you can argue that the Democrats, by not taking seriously things such as productivity and international trade, have found themselves in the same situtation.

Q. In your speech to the national convention of the Americans for Democratic Action last summer, you said, "The fact is that liberalism is at a crossroads. It will either evolve to meet the issues of the 1980s or it will be reduced to an interesting topic for PhD-writing historians, "which was pretty strong medicine to feed the ADA. How does it have to change? What do liberals do now?

A. What I have done is gone back and tried to figure out why movements take place. And let's use an analogy. If you ask yourself, why is a city located where it is, you will find that there are virtually no hap-hazard locations for cities. Things happen for a reason. The same is true with government and political parties. What was it that caused FDR to become a dominant philosophcal force for 50 years? If you go back and look at it, you find he embraced a certain set of humane values that was important but, above all, that worked. In some cases they were leftist policies, things that were considered socialistic at the time -- Social Security, for example. In other cases they were what would be today rightwing philosophies, like World War II.

And I think we have to do the same thing: We have to sit down in a very analytical fashion and try to determine what are the great issues of the next 20 years. Just simply identify them. What will be the forces that will move people? And if you can identify those, which I think you can rather easily, then you can figure out the best way of resolving our problems within a framework of certain human values. So what you end up with is not appealing to people for votes based on what we've done for the last 50 years but rather based on what we would do in the next 20. That is a very different approach.

Q: What are these issues for the next 20 years?

A: I'm now writing a book in which I deal with six of them. One is the issue of energy. Second is the issue of the Soviets. Third is the issue of Third World nations. Fourth is what I would refer to as biosphere overload -- people, pollution, that kind of thing. Fifth is the issue of the economic pie and sixth is the necessity of international trade competitiveness. I think liberals have a number of assumptions that are simply inoperative as you deal with some of these realities. We have been replaced by conservatives now, who come into these issues with a differing set of assumptions that are equally unworkable. We need a synthesis.

Q: But how do you compete for the hearts and minds of the voters with that complicated and long-run answer against a message like, "Let's get the government off our backs?"

A: You can't compete with it.

Q: Let's say you're turning for president and you buy a one-minute commercial and the first five seconds is, "I'm Paul Tsongas. I'm running for president. I'm a liberal." What do you say for the rest of it?

"A: My arguments cannot compete with simplicities.

Q: How do you win votes, then?

A: There's no way what I'm saying can possibly win votes head-to-head with Republican simplicites. There will come a time when the Republican simplicities have been tried and found wanting. At that point they will lose their appeal. There will come a point at which survival is a concern of 51 percent of the American people and at that point you have something to sell; not until that point.

Q: But that point isn't goint to be in 1982.

A: No.

Q: So what you're saying is, you all are going to be out in the cold for a good while.

A: Yes.

Q: What do you see as your mission in the Senate?

A: If I had control of what happened on Tuesday, I would not have allowed it to happen. The fact is it did happen. And now we're faced with two choices. One is to cave in, which I would have no objection to if the Republicans were right. If we were simply wrong and they were right, then we should cave in. But if you believe, as I do, that they suffer the same problems that our people suffer, then I think the task is to suggest a direction. You know, I look at my 3-year-old and think about the wild-eyed New Right itching for a showdown with the Soviets, and it makes me very angry. She should not suffer the consequences of that.

Q: It sounds to me like you're saying liberals should be philosophers for the next few years.

A: No, there will be enough liberals around to perform many different functions. But I see my major contribution being getting this sense of direction defined.

Q: Let's talk about the specific issues that might be involved in this sense of direction. For example: Do you favor price controls on oil and gas?

A: No.

Q: On anything?

A: On anything? Without having thought about each possible instance, my answer would be probably not.

Q: What if you can't afford to heat your home?

A: We passed the energy assistance bill, because we're going to have to meet those needs. But what good have you done anybody by having gasoline priced at a level that encourages the kind of automobiles that we have in this country? That maximizes our dependence on foreign crude? That exacerbates our balance of payments problems and puts us in a precarious national security situation?

Who have you served by doing that? If we have a shortfall, which is inevitable, who is going to get crushed in an energy crisis? Not the wealthy people. It'll be the poor people. To the extent you've avoided addressing that issue, you've insured their long-term non-survival. We have a fossil-fuel-based system, and it is a house of cards, and that seems to be of little concern to the Republicans.

Q: Do you believe in eliminating parts of the minimum wage laws, as Reagan does?

A: I voted against that.

Q: Do you believe it's important that the federal budget be balanced?

A: No. If you balance a budget at the expense of solving the basic underlying problems, like energy, you're living in a fool's paradise. Herbert Hoover tried to balance the budget. During World War II, if somebody had said, "Look, you can't spend money for arms to defeat Hitler, we have to balance the budget," they would have been put away. The imbalance of the budget at that time was astronomical. Everybody wants to balance the budget in someone's else's area. If you look at the Republican conservative senators who are the most outspoken in terms of a balanced budget and then look at issues like tobacco subsidies or water projects or the B1 bomber, they're saying, "Yes, balance the budget, but not on the things that I care about." I would like to see some kind of restriction on balanced-budget hypocrisy. I suspect we'd all be a lot better off.

Q: Do you favor a large-scale public service jobs program as an answer to unemployment?

A: I think it is the kind of thing you resort to when nothing else works. It is no substitute for the private sector. I come from an old mill city and I've been in politics for 11 years and I do a lot of work on bringing the city back and I always thought that the answer to it was federal funds and after all these years what we did eventually was understand that it was the private sector that would bring it back.

Q: So you strongly favor the growth of business?

A: If you have an expanding economic pie there is room for generosity. There is room for, if you will, liberalism. If you have an economic pie that has stopped growing, you have a very different emotional climate on your hands. That's why liberals who are intent upon a value system of generosity and compassion have the greatest stake in expanding the economic pie, the greatest stake in productivity issues. On the Chrysler package, when [Sen.] Dick Lugar [R-Ind.] and I got together and put together what we thought was a better bill, I found myself taking on the unions and liberals over the issue of productivity and wage freezes. For them, the first issue was maintaining certain values, certain traditions, even if the company went under.

Tax cuts are another example. Tax cuts generally come down to a fight between business tax credits and consumer tax credits. Democrats fight for the latter. Republicans for the former. It is not arguable that it is better in this economy to provide significant business tax credits to get the economy going? Consumer tax credits will simply add to the inflationary fires. Why are Democrats hung up on them? Why is it left to the Republicans to always sponsor the business tax cuts?

Q: You've just come out for a whole series of ways to help big business. You have in the past come out for nuclear power.How can you still call yourself a liberal? What's liberal about you?

A: I consider myself liberal on what would be termed social values: on issues like abortion, women's rights, gay rights, busing, affirmative action, that kind of thing. I'm opposed to capital punishment. I'm not the raw material of the Moral Majority. And I'm not going to back off on my views on those things. But you have to have both the realism that I argue for and deeper human values. If you end up with one without the other it won't work. It's got to be both in some kind of concert. The Republican approach won't work. The greatest danger to the Democrats as a party is a rational Ronald Reagan -- which would be good for the country. If Ronald Reagan turned out to be capable of stripping away his ideology he could be the new FDR, because it would work and there would be no reason to turn to the Democrats.

Q: Do you hope that happens?

A: As an American, yes. As a father of a child who would be obliterated by nuclear war, yes.

Q: As a Democrat?

A: As a Democrat, no. There is a conflict.