By Paul Kane

In a rare Saturday session, the Senate launched the final hours of debate leading up to a nighttime vote that serves as a critical early test for President Obama’s health-care proposal.

Shaping up as a cliffhanger, all but two of the 60 senators in the Democratic caucus have indicated their support for this early vote, which, if successful, serves to begin what will likely be several weeks of debate on dozens of amendments before a possible final vote before Christmas.

Majority Leader Harry M. Reid (D-Nev.) needs the last two holdouts – Sens. Mary Landrieu (La.) and Blanche Lincoln (Ark.) – to reach the 60-vote threshold to move forward with the debate. No Republican senator is expected to vote for this initial step.

In line with Lincoln’s demand for a 72-hour period to read the legislation, the key vote is expected at 8 p.m. Saturday, exactly 72 hours from the point Reid unveiled his health-care proposal Wednesday night.

Debate began at 10 a.m., with Sen. Patrick J. Leahy (D-Vt.) – a 35-year veteran of the Senate – discussing the decades-long effort to achieve national health insurance. Leahy quoted from Obama’s address to a joint session of Congress two months ago, just after their close friend, the late Sen. Edward M. Kennedy (D-Mass.), died after a lengthy fight with cancer.

Leahy urged his colleagues to pass the legislation in Kennedy’s honor.

“What we face above all is a moral issue,” Leahy said of Kennedy’s wishes.