The members of Congress are returning to a capital strikingly different in atmosphere from the one they left at the start of their recess. Death is in the air, and its sobering effects can be felt everywhere.

The summer ended with triphammer blows: the systematic shelling and killing of the Marines on "peacekeeping duty" in Lebanon; the murderous Soviet destruction of the Korean airliner, killing 269 persons, including Rep. Larry McDonald; and the sudden, unexpected death of Sen. Henry Jackson.

As is always the case, such tragedy has shamed the pettiness out of Washington politics. The atmosphere of soured partisanship Congress left behind when it took its August break has been transformed into a sense of shared concern and sobered realism.

President Reagan has been at his finest through this ordeal. Whether comforting the families of the slain Marines, expressing the nation's regrets at Scoop Jackson's death, or voicing the outrage everyone felt at the cold-blooded attack on the Korean airliner and its innocent passengers, his words, tone and demeanor have been exactly right.

Sometimes the president's communication skills have been ascribed to his long career as an actor. But this was no acting job.

The man who described the downing of the unarmed airliner by a Soviet fighter plane as "an act of barbarism, born of a society which wantonly disregards individual rights and value of human life and seeks constantly to expand and dominate other nations" was not a performer reading lines. He was a political leader speaking convictions. The president's actions were exactly what he said, in an earlier speech, the American response should be--"calm, controlled ad absolutely firm."

He saw to it that the Soviet Union was arraigned-- with damning evidence--in the court of world opinion at the United Nations. He reinforced the ban on the operations of its airline in the United States and encouraged other countries to join in measures ensuring the safety of international air travelers.

Isolating the lawbreaker and organizing the law- abiding to protect themselves is the appropriate response for a self-confident and civilized nation. "Vengeance," as the president said, "is not a proper answer." So he avoided the theatrical and politically cheap trick of calling off the arms control negotiations or expelling Soviet diplomats from this country.

What gave special poignancy to Reagan's impressive performance was that it so perfectly embodied the principles and even the personal style of Scoop Jackson, the man he mourned as "a true patriot."

Jackson was the least theatrical of public men. His friend and colleague, Sen. Daniel Patrick Moynihan, once described Jackson as having "the charisma of competence." The Philadelphia Democrats who heard Moynihan use that phrase during the Pennsylvania primary of 1976 did not know what he meant; indeed, most of the voters in most of the primaries Jackson contested in his failed bids for the presidency in 1972 and 1976 never did quite catch on to the special character of the man they saw.

Probably it took a long time for one to appreciate Scoop Jackson's qualities. I came to value them in 1960, when I saw him set aside his personal disappointment over the Kennedys' last-minute decision to pull back the implied offer of the vice presidential nomination when they decided Lyndon Johnson would be a stronger political asset. Far from sulking, Jackson accepted the lesser post of Democratic national chairman and worked his head off in the campaign.

That same year, he issued a report on national security organization, demonstrating the risks of letting a White House operator, the national security adviser, usurp the role and functions of the secretary of state.

He was right about that, just as history proved him right on other issues. Infallible he was not, but on most of the large questions, his judgment was awfully good. Most of all, he was realistic. He allowed himself few illusions, which made him especially valuable in a week like last week, when so many others found themselves disillusioned to the point of distraction.

"He lived in the worst of times," Moynihan said at last week's memorial service here, "the age of the totalitarian state. . . . He wanted his country strong because he knew the terrible danger of the age in which we live. Where others lurch from one issue to another with the attention span of a 5-year-old, he sustained this understanding and this vision through five of the most awful decades in the history of mankind."

Reagan was displaying those "Jacksonian" qualities last week--steadiness, strength and clarity--at a time when we needed them most.