THE CORPSE of the McGovern campaign, bloodbrowned and biodegrading, has been twice mashed into compost in the past week by the country's leading Democrat. With the subtlety of a retreating army, Senator Kennedy has abandoned hard-fought liberal positions in a drive to improve his position in 1976.

The party chairpersonship went first. The grand dragon of Democratic liberals--Jean Westwood--fell Saturday before the party's labor and oil forces, discreetly egged on by the senior Senator from Massachusetts. Westwood, who had reduced regular Democrats to fury in her pursuit of reformism, was replaced by Robert Strauss, a John Connally crony only loosely committed to quotes and other McGovern inventions. Strauss pointedly mentioned that his stewardship had Kennedy's approval.

On Monday night, Kennedy destroyed the McGovern campaign's line of attack on Nixon. "I, for one, will extend the olive branch to the Administration in the coming Congress." Kennedy said--a peaceful gesture to a man who McGovern had compared indirectly to Hitler. His prepared speech went further: it praised Nixon for "effective action" in the economy, and gushed that the trip to China brought the U.S. out of the "Dark Ages." Kennedy's statements, in contrast to the solidly strident noises of the 1972 campaign, seemed almost a rebuke to the McGovernites, a signal of a return to the old Democratic approach to political issues.

EVEN ASSUMING THAT Kennedy's "olive branch" to real remarks do not presage a lasting detente in the liberal Cold War against Nixon. As predictably as taxes fail to rise in election years, opposition candidates escalate their fire when campaigns accelerate. But the current lull in fighting is more than a ceasefire. It is a sweeping retrenchment, and it marks a huge change in Kennedy's strategy for the '76 election.

Massachusetts politicians now finally seem to appreciate the country's determination to avoid new leaps into corporate liberal programs. Kennedy plans to grasp the Democratic presidential nomination, but knows now that if he runs like McGovern he will enjoy the same success Kennedy thus must continue to play at campus fringe radicalism while maneuvering Democratic forces along pre-McGovern battlelines social security housing subsidies, welfare statism. He thinks he can win there, So Kennedy's side-burns go up the ducktail goes away, Westwood disappears beneath the boots of a Texas oilmen's lawyer and a tone of magnanimity enters criticism of Nixon.

But Kennedy's new approach is not without perils, George Romney and Ed Muskie know the traps waiting for the frontrunner. Kennedy may learn from their experience and he may not. McGovern once described Kennedy as a virtual prisoner of his staff, so presumably the staff, not he, will draw the conclusions.

EQUALLY OMINOUS to Kennedy's chances are the options open to George Wallace, who with Shirley Chisholm was probably the most honest, and least buyable, of the 1972 candidates. Wallace makes no effort to hide his distaste for liberals in general and Kennedy in particular. An Agnew-Kennedy choice in '76 would quite conceivably find Wallace pulling hard for the Republicans, given the seriousness of the Kennedy challenge. Any flat-out condemnation of Kennedy would send catastrophic tremors through Democratic ranks.

Last, Kennedy's campus and suburban liberal support may test out as limply as did McGovern's. In the latter case, numerous supporters, fed up by McGovern's prevarications and backdowns, dimmed in involvement. So they may with Kennedy. If voters perceive widespread Kennedy retreats on positions he was thought to have been previously locked into, they may come to agree with Harry Truman that, "No professional liberal is intellectually honest." Continued trampling of McGovernesque concerns may turn Kennedy's base into political sand.