The epic battle for health care reform gets its first big test on the Senate floor tonight, with Democratic leaders feeling confident, though not yet assured, of getting the 60 votes necessary to beat back a GOP-led filibuster.Senate Majority Leader Harry Reid (D-Nev.) has set a vote for 8 p.m. on whether to proceed to the debate on his $848 billion measure to overhaul the health care system.Republicans are attempting to persuade at least one Democrat to help them block debate on the bill, because their united force of 40 Members is insufficient to filibuster on its own. Sixty votes are needed to limit debate, or invoke cloture, on the motion to proceed to the measure, and Reid will need all 60 members of the Democratic Conference to succeed. If successful, a filibuster would prevent the measure from even being debated or amended on the floor.Though signs point to a positive outcome for Democrats this evening, two centrist Democrats – Sens. Mary Landrieu (La.) and Blanche Lincoln (Ark.) – have yet to declare how they will vote. Both have expressed concerns about the bill’s inclusion of a public insurance option from which states could opt out, and both are expected to announce how they will vote sometime Saturday.On Friday, another Democratic holdout, moderate Sen. Ben Nelson (Neb.), announced his intention to vote against the filibuster despite his misgivings on the public option, abortion language and other issues. He said he will work to amend the bill on the floor and may still opt to filibuster to end debate on the measure if the measure is not changed before final passage. Majority Whip Dick Durbin (D-Ill.) acknowledged the uncertainty of the vote on Friday but said Democratic leaders are hoping the chance to amend the bill will persuade wayward Democrats to at least allow the measure to come up for debate.“Sen. Harry Reid has done an enormously great job on this and has worked with Senators who were not certain they could join us in this debate. The bill reflects some changes that were made in that regard, but it isn’t over,— Durbin told reporters. “After this bill, I hope, is approved to go forward on the floor, there will be even more discussion. … [Senators will] raise issues that are important to them at home. We’re going to do our best to build a bill that is best for the nation and address some of their concerns.—To try to persuade Landrieu and Nelson to support the bill, for example, Reid included or omitted provisions important to their states, aides said. In Landrieu’s case, the bill will include increased Medicaid funding for states such as Louisiana that have been recovering from disasters. In Nelson’s case, aides have said, Reid declined to include a provision revoking the antitrust exemption for insurance companies. Nebraska is home to several insurance companies.But while Democratic leaders are stressing the opportunity to perfect the bill, Republicans are expected to try to put pressure the two holdouts by pointing out the potential impact on their states. On Friday, GOP Senators took to the floor to try to put some not-so-subtle pressure on Landrieu and Lincoln, who could face difficult re-election fights in 2010.In talking about the bill’s cuts to Medicare Advantage, Sen. Roger Wicker (R-Miss.) pointed out that estimates show an 81 percent reduction in Louisiana and a 40 percent reduction in Arkansas.Reid has avoided bringing up a House-passed health care reform measure because it includes several provisions opposed by both centrists and liberals in his caucus. The House bill contains abortion language opposed by Senate abortion-rights supporters and a public insurance option opposed by centrists.Though the Senate routinely takes House-passed measures and substitutes its own version over it, the vote today will be on whether to proceed to what Democratic Senators and aides have described as a “shell— bill – a House-passed measure to extend a homebuyer tax credit for military servicemen and women. However, Reid has already received the consent of Republicans to offer his substitute health care bill in place of that measure. Republicans had threatened to force Senate clerks to read that 2,074-page substitute amendment aloud, which would have taken several days of the Senate’s time and lasted well into the week of Thanksgiving, but they abandoned that threat in exchange for two full days of debate on the motion to proceed to the bill, Republican aides said.Just as it was on Friday, debate Saturday will be split evenly between Democrats and Republicans with each party controlling alternate hours of the Senate floor starting at 9:45 a.m. According to the bipartisan agreement, Democrats start the first hour of debate at 10 a.m. At 6 p.m., the debate is expected to get more heated as the parties go back and forth between opposing Senators every half-hour or so until the vote at 8 p.m.Steven T. Dennis contributed to this report.