* House Democrats near health debate, hunt for 218 votes

* Abortion opponents still threaten uprising against bill

By John Whitesides

WASHINGTON, Nov 4 (Reuters) - Democrats in the U.S. House of Representatives scrambled on Wednesday to iron out lingering concerns over abortion in a healthcare reform bill that was headed to a close and potentially historic weekend debate.

House Democratic leaders planned a Saturday vote on the sweeping overhaul, which would launch the biggest changes to the U.S. healthcare system since the creation of the Medicare health program for the elderly in 1965.

"We are on the verge of doing something great," House Speaker Nancy Pelosi, a Democrat, told reporters.

But with Republicans united in opposition, Democrats struggled to line up the 218 votes needed to pass the bill. "It's going to be tight," a Democratic aide said.

Democratic leaders hoped to defuse a potential uprising by moderates within the party who want stronger language to ensure federal funds are not used to pay for abortions under the bill.

The overhaul, President Barack Obama's top domestic priority, is designed to rein in costs, expand coverage to millions of uninsured and bar insurance practices, such as denying coverage to people with pre-existing conditions.