Another big birthday, this time the 200th of the Constitution, and where are we? Down in the dumps, in a rotten frame of mind, if you believe, as I do, that the members of Congress generally reflect the mood of their constituents.

On the first day after they returned from their summer vacation, Sen. William Proxmire (D-Wis.) led off a round of grumbling on Capitol Hill. During the recess, Proxmire announced his intention to retire next year, and you might have thought that would put him in a good mood. Think again. He was really unhappy.

''Mr. President,'' he began, ''the Congress of the United States always -- and I mean always -- takes a bum rap. . . . No one -- and I mean nobody -- ever defends the Congress. In more than 30 years in this branch of the Congress, and in literally tens of thousands of conversations back in my state, with people of every political persuasion, I have yet to hear one kind word, one whisper of praise, one word of sympathy for the Congress as a whole.''

This was no embittered veteran grumbling about the people who had turned him out of office. Proxmire is a political institution in Wisconsin. He has won reelection time and again with minimal opposition and no need for campaign funds. His unexpected retirement announcement provoked a bipartisan flood of praise.

But he told his colleagues he was ticked off by public reaction to the Iran-contra hearings, sympathy for witnesses he regarded as scoundrels and scorn for the legislators who were questioning them. Yet he said the chronic problem of Congress' low reputation can be blamed not on the Oliver Norths of this world, but on the members of Congress themselves. ''The people of this country think the Congress is the pits because we in the Congress tell them we are the pits. I have listened to many members of this body who have made a career out of attacking the Congress.''

That habit of demeaning our own competence and lowering our national credibility is also the subject of a worried editorial in The Economist, the British weekly that often seems to see the United States more clearly than we see ourselves. ''Whatever happened to America's smile?'' the editors ask in their cover story.

It was wiped off, they answer, by the realization that the United States is no longer the unchallenged military and economic strongman of the world. The frustration as real incomes have stagnated or declined for millions has deepened the gloom. They also surmise that many Americans have been ''embarrassed'' by the performance in the Iran-contra affair of the president in whom they had placed so much trust.

But even acknowledging all that, they say, there is no justification for the ''sulkiness, defensiveness and pessimism'' so evident to their eyes in America this autumn. They worry that because of this sour mood, the United States will turn inward -- toward protectionism in trade policy and isolationism or unilateral disarmament in foreign policy.

They may exaggerate, but anyone who has been traveling the country with the presidential candidates will recognize the portrait The Economist draws. Jack Kemp, by nature the most optimistic and expansive of the Republican hopefuls, has an edge of anxiety in his voice when he talks about eliminating the ''communist beachheads in this hemisphere'' or countering the maneuverings of Soviet leader Mikhail Gorbachev. As for the Democrats, a grimmer bunch of guys you'll never meet.

Patricia Schroeder, the only woman in the group, is also the only one who consistently displays the irreverent humor and the American gift for deliberate exaggeration that really get audiences laughing. Jesse L. Jackson offers a hope of salvation at the end of his speeches, as a preacher should, but the portrait of America he draws is an unhappy one of lost jobs and broken dreams.

Many of the others also talk about a declining, almost despairing America. Suicide is a recurrent theme in the speeches of Rep. Richard Gephardt, and even Michael Dukakis, who offers Massachusetts' 2.5 percent unemployment rate as a model of near-perfection to which the rest of America can aspire, does so in such a joyless way that you wonder if prosperity is all it's cracked up to be.

Maybe such a mood change was inevitable after the often mindless optimism of the Reagan years, when the president turned his back on jarring realities from the budget deficits to the exposed position of the Marines at Beirut airport. But I think The Economist editors are right in saying the sourness of the present mood, from Capitol Hill to Main Street, is uncharacteristic of the United States -- and unhealthy.

If nothing else, the bicentennial of the Constitution ought to remind us of the enormous flexibility and resilience of our underlying institutions. This nation has a long history of demonstrating its readiness to respond to energetic leadership and committing its resources to worthy struggles.

This country has not lost its knack for competing or for self-governing. So let's not talk ourselves into submission