At 2:30 a.m. yesterday, the Senate of the United States gave birth to a $606 billion baby called Continuing Resolution, or CR for short. The country will be fortunate if there is never again such a blessed event.

The House passed its version of the CR on Dec. 4 -- a modest little $576 billion infant, including 13 separate appropriations bills and a spate of unrelated legislation on broadcasting rules, environmental cleanup deadlines and a few other stray cats and dogs.

The Senate similarly bundled all 13 of its appropriations bills into its CR and added a batch of unrelated provisions, mainly bestowing favors on states where Democrats are seeking reelection.

When a conference committee of House members and senators has worked out the differences between the two versions of the CR, the final compromise will be dumped on President Reagan's desk for him to sign or veto as Congress flees town for the Christmas holidays.

Reagan says that the way the package is shaping up, he will veto it -- and he should.

This is, as House Minority Leader Robert Michel said, ''absolutely a lousy, rotten way to legislate.'' And that view is not confined to Republicans. Last summer, Sen. David Pryor said that CR really stands for ''combined retreat,'' or ''our admission of failure . . . at the end of each fiscal year, when we get ready to go home.''

Lumping everything together into one monstrous bill sharply reduces the ability of Congress and its members to make effective judgments on the nation's spending priorities. It also, and not accidentally, subverts the president's constitutional authority to veto legislation and have that veto count.

When everything from the Army's kitchen sinks to the National Institutes of Health's experimental drugs is wrapped into a single bill, passed in Congress' final hours of session, the president must either swallow it whole or accept responsibility for shutting down the government. Reagan says he will not shrink from the latter course -- but no president should have to face that choice.

This is a new and ugly feature of government, a phenomenon of the 1980s. Through most of its history, Congress has passed individual appropriations bills for individual departments or functions and sent them on to the president for his signature or veto. The CR was used only when a particular appropriation was briefly delayed and authority was needed for a department to go on spending for a short time.

But in recent years, Congress has fallen into the habit of wrapping all its spending authority into one CR -- and then loading it up with other measures to make them ''veto proof.''

Some blame the development on the new congressional budget process, which began in the mid-1970s, claiming it has slowed the work of the appropriations committees. But in the past couple of years, Congress has chosen to repackage even largely completed appropriations bills into the CR, rather than send them individually to the president for his approval or veto.

Increasingly, the CR has become a vehicle for shoving extraneous legislation down a reluctant president's throat. Earlier this year, Reagan vetoed a bill to reinstate the ''fairness doctrine,'' a regulation requiring broadcasters to present alternative policy views. The Senate failed by 13 votes to override the veto, so now congressional sponsors of the legislation have stitched it onto the House CR -- figuring Reagan would not veto money for military pay or school lunches just to win the fairness doctrine fight.

Whether you agree with Reagan or not on that issue, you have to recognize the Democratic architects of this strategy are attempting an end run, not just around Reagan, but around the Constitution.

What is to be done? When the Senate CR came up for action, Sen. Daniel Evans offered an amendment requiring that any future CR be split into its component parts when it comes out of the House-Senate conference committee, so that Congress can vote on each separate appropriation and the president can sign or veto each of them.

Evans lost by a narrow 51-to-44 margin. Similar legislation offered by Rep. Mickey Edwards was killed in the House Rules Committee, which denied Edwards the chance for a floor vote.

Edwards' sponsorship is significant. Unlike most other conservatives, he has consistently opposed Reagan's call for presidential authority to veto individual items in an appropriations bill. The ''line-item veto'' authority, Edwards has argued, would tilt the constitutional balance too heavily in the executive's direction.

But denying the president his right to receive and act on individual appropriations bills is a serious infringement on his constitutional authority, Edwards and Evans rightly say.

Whatever advantage Democrats may temporarily gain by using the CR device to thwart Reagan's veto, short-circuiting the Constitution ultimately endangers everyone. Congress should clean up its act before a new president takes office.