WASHINGTON — The deal is as politically remarkable as it is substantive: a long-term plan to finance health care for older Americans, pay doctors who accept Medicare and extend popular health care programs for children and the poor. It was cobbled together by none other than House Speaker John A. Boehner and Representative Nancy Pelosi, the leader of House Democrats, who rarely agree on anything, with the apparent blessing of a majority of their respective members.

Then along came a surprising impediment: Senator Harry Reid of Nevada, the minority leader, along with other Senate Democrats, objected to abortion restrictions in the bill and limits to an extension of a health insurance program for children. They have begun to undermine what was poised to be a sweeping bipartisan solution to several policy problems that have long vexed Congress.

It is a role in which Mr. Reid is becoming increasingly comfortable as he exploits his leverage in the minority to thwart his political opponents, even if that means an unusual split with Ms. Pelosi.

The House is expected to vote on the plan Thursday, but its fate in the Senate is unclear. The disagreements over the bill — aired in an unusually open fashion among leaders of the same party — were surprising and unsettling for Ms. Pelosi, whose staff members were privately questioning Mr. Reid’s strategy.