THE SEARCH FOR 60…. President Obama brought the entire Senate Democratic caucus to the White House, reminding the lawmakers how close they are to the health care finish line, and imploring them not to screw up this opportunity.

According to one of the senators, the president explained, “This is the moment of our legislative lifetimes. This is why people run for public office, to be here at the creation of something really big.”

The meeting became necessary in large part because Sen. Joe Lieberman (I-Conn.) was holding reform hostage. He spoke briefly at yesterday’s event, and according those in attendance, it produced this interesting exchange.

“What’s happening is not any fun for me,” Mr. Lieberman said.

[Sen. Sherrod Brown of Ohio], who has championed the public option, turned to Mr. Lieberman and said, “You know, Joe, it’s not fun for us either.”

At that point, Mr. Obama stepped in.

“Why don’t we all begin to have some fun?” he said. “Let’s pass the bill.”

Determining exactly how close Democrats are to doing just that isn’t as easy as it may sound.

The goal, obviously, is 60 votes to cut off a Republican filibuster. From the left, Sherrod Brown is on board, telling reporters yesterday, “I’m going to vote for it.” From the right, Sen. Mary Landrieu (D-La.) is also on board.

So, who’s left? Sen. Russ Feingold (D) of Wisconsin has not yet committed to the bill, and voiced his disappointment about the new round of concessions. Sens. Bernie Sanders (I) of Vermont and Roland Burris (D) of Illinois have also not said if they’re prepared to support the reform plan.

And then there’s Sen. Ben Nelson (D) of Nebraska, the caucus’ most conservative member, who had private meetings yesterday with both the president and Senate Majority Leader Harry Reid (D-Nev.). He told reporters after the White House event yesterday, “I’m not on the bill. I have spoken with the president and he knows they are not wrapped up today. I think everybody understands they are not wrapped up today and that impression will not be given.”

If Nelson backs the Republican filibuster, Democrats would need one GOP senator to break ranks in order to even have a vote on health care reform. Sen. Susan Collins (R-Maine) made it sound yesterday like she won’t be that vote, even if Democrats approve her proposed amendments.

And that leaves Sen. Olympia Snowe (R-Maine), who has been talking to Reid, but who seems — for unknown reasons — to want to push off a vote until 2010. Snowe did, however, vote for the Finance Committee bill, and there’s still reason to believe her vote is at least in play.

Majority Whip Dick Durbin (D-Ill.) said late yesterday, “We are moving toward 60 — we think we’ll have it by next week.” Stay tuned.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.