(Reuters) - As the healthcare battle moves to 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.
Here are some of the key Democratic moderates who are likely to play a big role. The Senate debate will not begin until Reid receives cost estimates on his bill and unveils the final legislation, which is expected in the next few weeks.
* 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 on Monday he 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.
* 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 on Sunday. His constituents in liberal and heavily Democratic Connecticut will be watching closely, but Lieberman does not face them again until 2012.
* 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.
* MARK PRYOR - The junior senator from Arkansas easily won re-election last year and is popular at home, easing his immediate political concerns about tough healthcare votes.
He remains publicly uncommitted on his stance on the procedural motions, but said he is keeping an open mind on the bill and a national government-run plan. "I'm open to the public option. It depends on how it's structured," he said.
* 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.
"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 in late October after Reid announced his support for a version of a public option.
* EVAN BAYH - 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.
Bayh, a popular former governor and son of former senator Birch Bayh, is expected to coast to re-election next year.