FACTBOX: Centrist U.S. senators hold key to healthcare fight

(Reuters) - As the healthcare battle moves into the U.S. Senate, a small bloc of centrist senators hold the key to reaching the magic number of 60 votes needed to overcome Republican procedural hurdles and pass a bill.

Democrats have little margin for error -- they control exactly 60 votes, meaning the defection of even one could block President Barack Obama’s top domestic priority.

Some of those centrists -- mostly from conservative or Republican-leaning states -- have voiced doubts about backing a plan that includes the government-run public insurance option outlined by Senate Democratic leader Harry Reid.

These senators will be watched closely on two procedural votes -- to allow debate to begin on the bill and to allow a final vote on the bill.

If the Senate takes up the bill, the debate is expected to begin on November 30, after the U.S. Thanksgiving holiday next week, and last for at least three weeks.

The first test vote on whether to begin debate on the bill is expected by the end of the week. Here are some of the key Democratic moderates who are likely to play a big role.

* BEN NELSON - Often viewed as the most conservative Democrat in the Senate, he is uncommitted on whether he will side with Republicans and try to block either debate or a final vote until he sees the legislation that Reid brings forward and the cost estimates for it.

He has been outspoken, however, in opposing the national government-run insurance option Reid has proposed -- he favors a state-based approach -- and earlier this month said he backed the strengthened restrictions on federal funding for abortions adopted by the U.S. House of Representatives.

Nelson does not face re-election in conservative Nebraska until 2012. But voters will remember what he does. If they don’t, Republicans may likely remind them.

* JOE LIEBERMAN - An independent who caucuses with Democrats, he has not been a reliable vote for the party since his support for the Iraq war led to his defeat in the 2006 Democratic Senate primary. He won re-election three months later as an independent, and further angered the party by backing Republican John McCain in the 2008 White House race.

Lieberman repeatedly has said he will join Republicans to block a final vote on the bill if it includes a public option, but will not block the general debate.

“If the public option plan is in there, as a matter of conscience, I will not allow this bill to come to a final vote,” he said last week. His constituents in liberal and heavily Democratic Connecticut will be watching closely.

If Lieberman helps stop healthcare reform, he would face new calls to strip him of a coveted committee chairmanship as well as a spirited Democratic challenge when seeking re-election in 2012.

Reid, who faces a tough re-election bid next year, has indicated he expects to resolve differences with Lieberman on the legislation. To do so would help both politically.

* BLANCHE LINCOLN - She has a potentially tough re-election fight next year, giving her incentive to avoid votes that could cause political heartburn at home in conservative Arkansas.

She backed the final healthcare bill in the Senate Finance Committee, but during the debate she voted against two amendments to add a public insurance option.

Like Nelson, she is uncommitted on the procedural votes until she sees a bill and its cost estimates. But she sounds open to a compromise that could incorporate Republican Olympia Snowe’s proposal for a “trigger” to activate a public option in areas with inadequate competition in the insurance market.

* MARY LANDRIEU - The third-term senator says she is concerned about the Senate’s public option plan but is amenable to a compromise, particularly one involving Snowe’s trigger idea. She won re-election in conservative Louisiana last year despite being heavily targeted by Republicans.

What she does on the healthcare bill may follow her to 2014, when the impact of any enacted legislation would have kicked in -- and she will again be up for re-election in Louisiana, a state that leans Republican.

“I am encouraged that the conversations taking place over the past week among senators who back different versions of a public option could potentially lead to a compromise,” she said last month.

* EVAN BAYH - A popular former governor and son of former senator Birch Bayh, he’s seen as a heavy favorite to win re-election in Indiana next year. But as a centrist in a Republican-leaning state who may someday run for the White House, Bayh must tread carefully. His father lost his 1980 bid for a fourth Senate term largely because he was seen as too liberal.

Bayh says he is focused on restraining healthcare costs for working families under the bill and bringing down its costs, rather than the battle over the public insurance option.

He initially said he would treat the procedural votes the same as the substantive votes -- meaning he would block debate and a vote if he decides he opposes the bill. He is now expected to vote to allow debate to start, although his stance beyond that is unclear.

Writing by John Whitesides and Thomas Ferraro; editing by Arshad Mohammed and Mohammad Zargham