Depending on the outcome of the recount in Minnesota, Democrats will either wind up with a 58-seat caucus in the 111th Senate or a 59-seat one. Therefore, they will be either one or two votes short of the 60-member majority required to pass a cloture motion and end a filibuster, assuming that everyone votes along party lines.
But of course, not everyone will vote along party lines. We have identified at least five moderate Republicans in states won by Barack Obama who, for one reason or another, will be under pressure to vote with the majority. Conversely, there will be at least four moderate Democrats in states won by John McCain that will be under pressure to vote with the Republicans.
The theory is this. Imagine that you're a moderate Republican, like Olympia Snowe of Maine, and you're contemplating how to vote on a measure like the Employee Free Choice Act, which ought to be just on the threshold of that 60-seat majority. You have conflicting incentives here: perhaps your constituents support the bill, but your party caucus certainly does not; you might have mixed feelings about it personally.
There are four possible outcomes to this dilemma:
a. You vote for the bill and it passes.
b. You vote against the bill, but it passes.
c. You vote for the bill, but it fails.
d. You vote against the bill, and it fails.
Which of these four outcomes is the most desirable for you? Probably this one:
You vote against the bill, but it passes. This way, your colleagues in the Republican caucus will be happy. But, your constituents will probably be reasonably happy too. They get the piece of legislation they wanted, and in time, your nay vote will almost certainly be forgotten about. And your risks are pretty well hedged: if the bill becomes unpopular later on, you can always remind the voters that you were against it.
It seems to me, however, that if everyone thinks like this, and if the Democrats have exactly 59 votes, then you run into something of a NIMBY problem. Namely, if it takes just one Republican vote to pass a certain measure, then why does it have to be yours? Why can't your colleague Susan Collins vote for it instead -- or George Voinovich, or Arlen Specter? (And of course, Susan Collins and George Voinovich and Arlen Specter are thinking exactly the same thing about you.) If the Democrats have just 58 votes, on the other hand, you know that at least two Republicans will have to vote to approve the measure, and so you cannot rely solely on someone else taking one for the team.
A related principle is probably this: if the bill passes, you would probably rather it pass with as many Republican votes as possible. This gives you more cover with your party leadership: they have any of several people they can blame, as opposed to you and you exclusively. Being the only member of your party to vote for the bill, on the other hand, could be pretty terrifying. But this too becomes tricky if the Democrats have exactly 59 votes. If the Democrats have 58 votes, then you know the cloture motion will not pass without you and at least one other Republican voting for it. If they have 59 votes, on the other hand, then your vote and yours alone could make the difference.
My point, of course, is that the psychology could be surprisingly different if the Democrats have 59 votes as opposed to 58. In the former case, it's everyone out for herself, and you risk creating some sort of multi-way Prisoner's Dilemma, in which all the moderate Republicans bank on all the other moderate Republicans supporting the bill. The latter case, on the other hand, necessitates collaboration and collective action.
Now, there are a number of obvious critiques to this theory, most notably that it will rarely be the case that the Democrats are guaranteed to have exactly 59 (or 58) votes in hand, since any of several moderate Democrats could plausibly break with the majority on any given measure. Still, it is easy enough to imagine a case where one is the most dangerous number to Democrats. I suspect that 59 is in fact a better number for the Democrats than 58, but perhaps by a surprisingly small margin.
Addendum: Another lens to view this through is as follows: it may be easier for the Republicans to maintain party discipline when they know that every single member of their caucus will need to vote against a cloture measure in order to defeat it.