A handful of centrist Senate Democrats, along with moderate Republican Sen. Olympia Snowe, could pose a major problem for Senate Majority Leader Harry Reid as he tries to pull health care reform legislation to the floor. 

Snowe, in concert with Sen. Ben Nelson, D-Neb., has for weeks held meetings with a group of five or six moderates, including Sens. Joe Lieberman, D-Conn.; Mary Landrieu, D-La.; Blanche Lincoln, D-Ark.; and, at times, Susan Collins, R-Maine. 

They do not all appear to hold the same view on health care reform. But several members of this group have expressed concern with the possibility that a government-run insurance plan could emerge from closed-door talks in the majority leader's office -- though they are not outright opposed to a public plan. 

And they are very aware that a procedural motion known as "cloture" -- which requires 60 votes to bring legislation to the floor -- could be their best opportunity to stop any bill they find objectionable. 

Sen. Kent Conrad, D-N.D., and a number of senior Democratic officials have said a national public option that gives states the ability to opt out if they can provide substantially competitive coverage to the uninsured and underinsured is becoming more popular in negotiations among Senate Democratic leaders and White House officials. 

Snowe, who also has met regularly with administration officials, called the emerging proposal "just another public option." 

"I do not support it," she said. 

Snowe went one step further and warned that any inclusion of this "opt out" measure would cause her to withhold her support from proceeding to the bill. 

Her "no" vote alone would not be enough to stop the bill from moving forward. But in what could be an ominous sign for Reid, Nelson called the "opt out" public plan a "difficult option" and questioned how difficult it would be for states to opt out. 

"I've not said my cloture vote is a given here," he warned. 

Another big concern in the group is whether they will see the bill before debate starts. 

Snowe said she wants to examine and "hold tutorials" on what's in the bill before it even heads to the Congressional Budget Office, the nonpartisan agency that analyzes and tallies the price of legislation. 

Snowe said her goal, shared by some of her centrist colleagues, is "not to rush this train out of the station." 

The senator pointed to a vote Reid lost on Wednesday related to Medicare doctor reimbursements, through the same kind of procedural motion that will come on a broader health care reform bill, as "a warning" to Reid, saying that could easily happen on the bigger bill. 

The Senate moderates are in the middle of a tug-of-war. Liberal senators who really want a robust government-run insurance plan have said that those who are opposed to it should just get out of the way on cloture and then vote "no" on final passage, when Reid only needs 51 votes to succeed. 

Reid has said he would make the bill and a full CBO score available to members before bringing it to the floor.