WASHINGTON - With no margin for rebellion, Senate Democrats yesterday pushed toward a crucial weekend test vote on their sweeping health-care bill, and wavering moderates appeared to be falling in line on President Obama’s signature agenda item.

One of three uncommitted centrists, Sen. Ben Nelson of Nebraska, announced he’d vote with his party’s leaders on today’s must-pass procedural measure allowing debate to go forward.

Nelson said it didn’t necessarily mean he’d back the final bill but did mean Nebraskans wanted changes to the health-care system. “The Senate owes them a full and open debate,” he said.

The nearly $1 trillion, 10-year bill would extend coverage to millions of uninsured Americans, bar practices such as denying coverage to people with pre-existing conditions and require most Americans to purchase insurance.

Sixty votes are required to clear Saturday’s vote, meaning all 58 Democrats and two independents who usually vote with them need to hold together. All 40 Republicans are united in opposition.

“We are not assuming a thing,” said Dick Durbin of Illinois, the No. 2 Senate Democrat. “I’m hoping that we can muster our ranks.”

Nelson has been one of just three question marks in recent days, along with fellow moderate Democratic Sens. Mary Landrieu of Louisiana and Blanche Lincoln of Arkansas. Landrieu signaled she’d vote to allow debate, while Lincoln held out.

Meanwhile, Republicans pushed back. Sen. Jon Kyl, (R-Ariz.), noted at a news conference that national polls show at best a split on the health-care bill. “We hope our more moderate colleagues on the Democratic side would respect the wishes of their constituents, rather than do the bidding of Harry Reid,” Kyl said.