Democrats are quietly blocking Senate consideration of President Reagan's nominations for 40 new federal judgeships in hopes of reserving at least some of the new judicial slots until after the November election.

Four years ago, it was the Republicans who blocked many of President Jimmy Carter's election-year judicial nominations. Now the Democrats are doing the same thing, although a compromise is expected to be reached that would allow some of Reagan's judgeship choices to be confirmed before Congress quits for the year early next month.

Aides to the Senate Republican leadership said Majority Leader Howard H. Baker Jr. (R-Tenn.) and Minority Leader Robert C. Byrd (D-W.Va.) plan to meet shortly to see if a formula for pre-election and post-election consideration of the nominations can be worked out.

In all, Reagan has about 45 district and circuit court judgeships he could fill before the end of his current term, including 40 new positions created in recently enacted legislation on bankruptcy court reform.

The number of new judgeships for this year was cut by half in a House-Senate compromise on the bankruptcy bill, under which 85 new judgeships were created with 45 of them reserved until after the end of Reagan's current term.

Under Senate procedures, however, the Democrats are able to block even the 40 designated for this year, and shortly before the congressional recess for the Republican convention last month they served notice that they would do so.

The standoff contributed to cancellation of a Senate Judiciary Committee meeting yesterday, although committee aides said Democratic objections to consideration of the judicial nominations was only one factor in the committee's inability to muster a quorum. They said a main factor was the delayed return of many senators from the recent recess.

Cancellation of the session meant another delay in committee consideration of Reagan's proposal for a constitutional amendment to require a balanced budget, which some Democrats also are seeking to block.

The proposed amendment also is bottled up in the House. Responding to a claim by House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) that the House would act within 48 hours on a balanced budget if Reagan would only send it one, Minority Leader Robert H. Michel (R.-Ill.) said yesterday that O'Neill should put together a balanced budget of his own if the House is so ready and eager to act.