The Senate, for a fourth time, failed yesterday to cut off a filibuster against Democratic-sponsored legislation to limit spending for Senate campaigns, and Majority Leader Robert C. Byrd (D-W.Va.) said he was preparing another compromise aimed at picking up Republican support.

The vote was 50 to 47 to invoke cloture to limit debate on the measure, 10 votes short of the 60 necessary for cloture. With all senators present, Byrd would still be seven votes short, roughly where he was when debate started on the measure two weeks ago.

But Byrd indicated yesterday that he will stick with the issue indefinitely, putting the Senate on notice to expect late sessions next week to consider the campaign-financing measure, perhaps in conjunction with other legislation such as the huge trade bill.

Rollcall votes "can come early . . . or late . . . or late, late, late," Byrd said, although he indicated he would skip a cloture vote Tuesday in hopes of breaking through the partisan deadlock with a compromise offer that he said might prove "attractive" to at least some Republicans.

The legislation before the Senate would offer partial public financing of campaigns to candidates who accept spending limits as part of a broad effort to curtail campaign costs and curb special-interest influence on elections.

An earlier offer from Byrd to reduce the amount of taxpayer financing failed to budge the Republicans. His newest offer is expected to reduce public financing even further, possibly by triggering its release only when a candidate's opponent refuses to accept spending limits.

Republicans have objected both to spending limits and to public financing, but have been especially acerbic in their criticism of using taxpayer funds to finance congressional elections. They have rallied around legislation of their own to limit or ban contributions from political action committees (PACs). The Democratic measure also would limit PAC gifts.