Every time the conventional wisdom gets practically unanimous in Washington - "Truman is done" (1947-48), "Johnson can't be toppled" (1968), "Muskie is a shoo-in" (1972) - things are fairly sure to turn out quite the opposite. So it doesn't take much courage to attack the present conventional wisdom that liberalism is dead or at least dying.

Actually, the most significant vital signs point the other way - that liberalism is quite alive and well:

While columnists and editorial writers prate about failings of New Deal and Great Society social programs, the public demands one of the biggest social programs of them all, national health insurance, and the present political struggle is not over whether there should be such a program but over the time period necessary to phase it into the economy.

With all the cannonading for tax reduction and spending restrictions, the attack never focuses on any particular New Deal or Great Society program. The Proposition 13 and the Kemp-Roth backers don't level their fire at particular programs to be curtailed or repealed, but rely for support on Pavlovian antipathy to the very real unfairness of the Present tax system. Despite all the demagoguery calling for a rollback of big government, the "welfare state" social programs are so deeply imbedded in the system that no one seriously proposes their curtailment or demise.

On the civil-liberties front, even so hateful a group as the Nazis was given full First Amendment protection for the recent march in Chicage, and the bill for a revised criminal code, supported by most of the Senate's most distinguished liberals, is being held up in the House not because it is too liberal but because leading members of the House feel it doesn't do enough to rectify existing law with anti-civil-liberties aspects.

On the civil-rights front, the Supreme Court, often a bellwether of the public mood, used its Bakke decision not only to validate race-conscious preferences for education, employment, etc., but to announce that goals and timetables and even quotas are within constitutional sanction where there are legislative, judicial, executive or administrative findings of past discrimination to back them up.

The Moynihan-Meany-Jackson hardliners have, thus far at least, had only minimal success in their attemps to reopen the cold war and this despite considerable Soviet provocation.

Many large labor unions are coalescing with liberal groups more closely than they have for years.

The nation's conservative party, the Republicans, have made little if any progress in broadening their ranks.

Every poll shows Sen. Edward Kennedy (D-Mass.), who has a near-perfect voting record over the years on the Americans for Democratic Action and other liberal scorecards, far and away the No. 1 choice of Democrats for 1980.

Of course, a number of problems for liberals do inhere in the current political situation. For example, many of today's issues are incremental ones extending or improving upon the more fundamental economic reforms of Franklin Roosevelt or civil-rights reforms of Lyndon Johnson and thus far less easy to dramatize than earlier more fundamental changes. Then, too, 45 years of social change in the interest of a more egalitarian society was bound to produce a certain amount of the tired "we-have-done-enough for them" syndrome and the self-interested "we have to be with the winner this time" syndrome.

And, finally, the hardest time for liberals is when there is a middle-of-the-road Democrat in the White House. It is easy to work in support of a liberal Democratic president. It is not too hard to work in opposition to a conservative Republican president. But a non-ideological middle-of-the-road Democrat in the White House creates a sort of noman's land with maximum uncertainty as to who is doing what to whom.

But whether the next surge of liberal successes comes from a left-turn in the White House (Truman made his big shift leftward as the 1948 election neared), or from the fine young liberals in the administration banding together to demand more vigorous action,or from the deep belief of the American people in the role of government as the builder of an ever-fairer society, conservatives are clucking much too soon.