1982 Republicans defending only 13/33 seats.
1984 Republicans defending 19/34 seats.
2010 Republicans defending 19/34 seats.
2012 Republicans defending 9/33 seats.
I think it's pretty clear that, almost regardless of what else happens, and no matter how big Obama wins in 2012, the Democrats are going to lose seats in the Senate that year, probably enough to knock them down below 60 votes, maybe significantly below 60 votes. The key is 2010. If the unemployment rate (and for reasons I won't go into here, I believe almost nothing else will matter on the economic front) is declining, the Democrats pick up two, maybe three seats in the Senate, and then they can do almost what they please. If the unemployment rate is stagnant, or, worse case scenario, climbing, they stay flat to +1. As was true in 1984, when Reagan won huge with a historically high, but improving, unemployment rate, what matters is the sense of progress. Obama has done a good job of painting a sufficiently ugly picture of economic reality; now he has to show he can improve that picture.
I think the still-fresh memory of Republican failure, and their astonishing tone-deafness to date, plus the disadvantage of defending more seats, makes it highly unlikely they will actually pick up anything in 2010. The goal, for them, should be avoiding losing too many, just as the goal for the Democrats in 2002 - 2004 was to avoid losing so many they lost the ability to filibuster -- not that they used that ability all that often or well.
The point of all this is simple: Obama has, at the very most, three years to pass major legislation, like universal healthcare. The odds are he's going to be sitting pretty after 2010, but he's shot himself in the foot some with the too-small stimulus package, which means the economy could still be shedding jobs just when we're hitting the 2010 election. If that happens, a lot of the initiative Obama enjoys now will evaporate; he'll probably lose some of the cooperation of the crypto-wingnuts Snowe, Collins, and Specter; and face rebellion from the Blue Dogs in his own party, who'll be looking to distance themselves from "failure" in a (futile; as we saw in '02 and '04, when the winds blow against liberalism the moderate Dems in Red states are the first to go no matter how obsequious they are) attempt to save their own political skins.
The upshot of all this is that the Dem position is strong now, but it is hardly impregnable. The Republicans should be fighting a holding action to get safely to 2012, and then they will be in a much stronger position to obstruct. The way to have taken that away from them was to make certain the stimulus package was big enough all but to guarantee success, get a big working majority in 2010, and then enact popular and necessary legislation regardless of Republican whining. But the stimulus wasn't big enough to guarantee success. And now, we have to hope.
One annoying thought at the back of my mind: some time around late spring in 2010, the economy is still failing, what does Obama do? Public works-type spending won't hit the economy fast enough to move the dial on the '10 elections, the Republicans plus quaking Blue Dogs will block such legislation anyway, so he's left with one form of fiscal stimulus: tax cuts. The Rushpublicans would crow that they were right all along, that spending doesn't work, Keynes was an idiot, blah blah blah. It's a disaster that could easily happen. So much depends on the economy showing life in the next 18 months....