A key Democratic senator on the Appropriations Committee is quietly passing word to colleagues to "resist at all costs" the predictable effort of committee Republicans to boost the defense appropriations bill. That bill is due for committee markup soon after the rump session of Congress convenes this week.

Here is the first critical test for Democrats in a Senate that comes under Republican control in January: Will they follow the spirit of bipartisan cooperation proclaimed by Jimmy Carter and Ronald Reagan for the transition period? Or is their anti-defense mind-set so fixed in concrete that, in effect, they will exploit the lame duck session and their disappearing majority to repeal the Nov. 4 election?

The quiet word being passed out of the office of just-reelected Sen. Dale Bumpers of Arkansas to colleagues and Pentagon lobbyists leaves little doubt about the answer. Instead of bending to the big blow of Nov. 4 that decimated the anti-defense ranks of Senate Democratic liberals, the answer is a resounding "down with defense."

That atmosphere is quite different from the surface mood of bipartisan back-scratching set by a magnanimous post-election Jimmy Carter and a Ronald Reagan thankful for small favors from the outgoing administration. Whether it lasts or not, this mood remains pervasive in the immediate election afterglow. One member of Reagan's transition team pointedly declined a "juicy inside tip" from a Republican mole in the Pentagon casting doubt on the Stealth aircraft, which Carter's men see as America's salvation 10 years hence.

The contrasting mood of politics-as-usual on Capitol Hill points to the Senate as the scene of the first great post-election struggle over an issue that, as much as any other, symbolizes the cleavage between Carter and Reagan, liberals and conservatives and the old and the new Congress: national defense.

Confident of his own reelection last summer, Carter worked behind the scenes to prevent Senate action on the $157 billion House-passed defense bill. Delaying it until after the election would give him the option of a possible veto, plus the clout of a new four-year term to influence voting in the Senate.

The decisive nature of the defeat suffered by both Carter and liberal legions in the Senate has now put the long-suffering bloc of defense-minded senators on top -- but not until the new Congress comes to town in January. To build a modest defense foundation that will be radically expanded next year, Reagan transition planners want the Senate to pass the House bill with or without amendments before adjourning for Christmas.

But some Republican leaders of the defense bloc are dreaming larger dreams. They will push hard to increase the bill by about $4 billion in new weapons authorized by Congress but not financed in the House bill. If these increases are voted down, as Bumpers wants, the Republicans privately threaten to filibuster final passage of the bill, preferring a simple resolution to permit continued defense spending at current levels to a truncated bill for the year that started Oct. 1.

But that approach invites political disaster for President Reagan. If the defense money bill fails to pass this year -- after the long, intentional delays visited on it by the Carter administration -- a brand new start must be made in the new Congress next year.

Down that road lies the danger of unacceptable delay. Two defense bills -- one for the year already started and one for the new fiscal year -- would have to be sent to Congress together, with Rep. Joseph Addabbo of New York, a bonded liberal, in the driver's seat as chairman of the House Defense Appropriations Subcommittee. It is almost inconceivable that the first of those two bills would get through Congress before July.

Such sorry fiscal management of the national defense should be unthinkable. If the Democrats who survived Nov. 4 show a particle of the spirit of compromise promised by President Carter, the long-delayed defense bill now stalled in the Senate will be rushed to the president's desk with the additions Reagan needs.

If not, the defense bloc can only bide its time by voting whatever it can get for now, sending a truncated bill to President Carter before adjournment. The long, expensive buildup toward Reagan's promised "margin of safety" will then start in January, but at least the foundation would be in place for a supplemental appropriation and a new defense bill for 1982.