WASHINGTON  Now it is Senator Harry Reid’s health care bill.

The majority leader’s deep personal involvement in assembling the overhaul of the health care system  now on the brink of a historic Senate debate  has, for better or worse, conferred ownership of the legislation on Mr. Reid, Democrat of Nevada.

Should Mr. Reid shepherd the measure successfully through the Senate and meld it with the House version into final legislation that President Obama can sign, it would be the biggest accomplishment of his career. Should the bill fall victim to the complex political, procedural and substantive fights raging around health care, it would be a stinging defeat for him, his president and his party  all while he faces a tough re-election fight at home.

Colleagues say Mr. Reid’s extensive knowledge of Senate tactics and well-honed understanding of what drives and divides his Democratic colleagues leave him well positioned to pull off a legislative coup that has eluded seasoned and determined lawmakers for decades.

“I don’t think there are many people in the whole world other than Harry Reid who could do this,” said Senator Mary L. Landrieu, Democrat of Louisiana, one of the lawmakers who have been a thorn in Mr. Reid’s side with their reluctance to commit to supporting a preliminary vote to open debate. That vote is set for Saturday evening.