WASHINGTON -- Senate Democratic leaders achieved a breakthrough Saturday in the drive for health overhaul legislation, securing the 60 votes needed to ensure passage after a late-night deal on abortion coverage locked in the support of Nebraska Sen. Ben Nelson.

The agreement capped weeks of negotiations aimed at building consensus on the White House-backed initiative. It sets the stage for a vote Monday, possibly at 1 a.m., on a pivotal motion to close off debate on the bill. Senate Majority Leader Harry Reid is expected to...