Washington ? In a major breakthrough, union leaders bowed Thursday to White House demands for a new tax on high-cost insurance plans as part of landmark health care legislation taking final shape in intensive negotiations. “We are on the doorstep” of success, President Barack Obama said.

The tentative agreement on the tax, which included significant concessions by the administration, was disclosed as leading lawmakers set an informal timetable of today for a compromise on the health care bill that Obama made a top priority in taking office a year ago.

Democrats expressed the hope that the agreement would quickly open the way for progress on other key issues where House and Senate-passed bills differ, as well as attempts by the White House to squeeze additional financial concessions from drug makers, nursing homes and other health care providers.