Washington (CNN) -- The first crucial showdown over health care reform by the full Senate could come as early as next Tuesday.
That's when Senate Majority Leader Harry Reid hopes the chamber will vote to start debate on health care legislation. Though Reid put the wheels in motion for next week's vote, nothing is guaranteed.
Democrats need 60 votes to pass the motion to start debate. While there are 60 members in the Democrats' coalition, Ben Nelson, a moderate Democrat from Nebraska -- says he hasn't decided which way he'll vote, and won't until he sees the actual bill.
In fact, no one has seen the Senate health care bill yet. Reid won't release the legislation until he knows the cost of the bill.
Reid, a Democrat from Nevada, has been waiting nearly three weeks for the nonpartisan Congressional Budget Office to return its score, or cost estimate, of the bill. Reid aides say they expect the CBO information on the cost of the bill by the end of this week.
But the second-ranking Democrat in the Senate, Dick Durbin of Illinois, said Congress likely will miss President Obama's deadline to enact health care reform by the end of the year.
The House voted 220-215 on Saturday to pass its version of health care reform. Durbin said he hopes, at best, to pass the Senate's version of a health care bill by that time.
But legislation that emerges from Senate would then have to be reconciled with the House bill.
Durbin's assessment came as former President Clinton made a rare visit to Capitol Hill to discuss the health care issue with Senate Democrats.
Clinton tried but ultimately failed to pass a health care overhaul in the early 1990s. The failure is considered one of the reasons for the GOP takeover of Congress in the 1994 midterm elections.
"Whatever their differences are, I just urged them to resolve their differences and pass a bill," Clinton said on Capitol Hill. "I also believe, you know, people hired us to come to work in places like this to solve problems and stand up and do it."
Clinton told the senators that they had a once-in-a-lifetime opportunity to push through a reform package that he said is crucial to the country's long-term economic health.
Durbin blamed the shifting timeline on a slower-than-expected cost analysis from the CBO on legislation being crafted under Reid's direction. He also cited an uncertain schedule for floor debate.
Durbin said he hopes debate in the full Senate will begin before Thanksgiving.
Among the many obstacles Reid is facing is whether to include a government-run public option. The majority leader has proposed including an option that would give states until 2014 to decide whether to opt out. But several Democratic moderates -- as well as Sen. Olympia Snowe of Maine, the lone Senate Republican to vote for reform -- have expressed deep reservations over the idea.
CNN's Ted Barrett, Dana Bash, Paul Courson, Jill Dougherty, Brianna Keilar, Paul Steinhauser and Deirdre Walsh contributed to this report.