WASHINGTON - One of three wavering U.S. Senate Democrats said on Friday he would vote to start the chamber's healthcare debate, bolstering the chances for a broad overhaul one day before its first crucial test.

Senator Ben Nelson said he would support a procedural motion on Saturday to allow debate on healthcare reform to proceed, even though he is still unsure if he will back the final bill.

Throughout my Senate career I have consistently rejected efforts to obstruct. That's what the vote on the motion to proceed is all about, he said in a statement.

It is only to begin debate and an opportunity to make improvements. If you don't like a bill why block your own opportunity to amend it? he said.

Democratic Senators Blanche Lincoln and Mary Landrieu remain uncommitted on the vote to proceed to debate on the overhaul of the $2.5 trillion healthcare system, President Barack Obama's top domestic priority.

The procedural motion requires 60 votes to pass in the 100-member Senate and Democrats have no margin of error -- they control exactly 60 votes.

Republicans so far are united in opposition to the plan unveiled on Wednesday by Senate Democratic leader Harry Reid, which is designed to rein in costs, expand coverage and bar industry practices like denying coverage to those with pre-existing conditions.

It would be our hope that our more moderate Democratic colleagues would respect the wishes of their constituents rather than do the bidding of Harry Reid, Republican Senator Jon Kyl told reporters.

If the Senate takes up the bill, the debate would begin on November 30, after next week's U.S. Thanksgiving holiday. It would last at least three weeks, making it unlikely Obama can sign a final bill by the end of the year.

Any differences between a Senate bill and the House of Representatives version passed on November 7 would have to be reconciled before a final bill can be voted on again in both houses and sent to Obama.

The overhaul would lead to the biggest changes in the U.S. healthcare system -- which accounts for one-sixth of the U.S. economy -- since the 1965 creation of the Medicare government health insurance program for the elderly.

Nelson, Landrieu and Lincoln, moderates from conservative states where the healthcare overhaul is not popular, have rebelled at Reid's inclusion of a new government-run public insurance option in the plan.

Senator Dick Durbin, the No. 2 Democrat in the Senate, said Lincoln had told Reid how she would vote on Saturday. That would indicate she is likely to support it, or Reid would not have scheduled the vote.

Reid and the Senate's other Democratic leaders, hoping to avoid an embarrassing setback for Obama, have been pushing the wavering senators hard to allow the opening of the debate, arguing that they can seek to change the bill as it unfolds.

(Editing by Vicki Allen)

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