Aside from the reasons that Sen. Obama outlined in his official response on the issue of FISA and telecom immunity etc., I honestly believe that what the Dems and Obama fear the most is another terrorist attack between now and the November election. Politically speaking, supporting the FISA bill is a C.Y.A move all the way.
God forbid there is another terrorist attack, you know it, I know it, we all know it - the Republicans will scream from the top of their lungs that Obama and the Dems stood in the way of important legislation to keep Americans safe. No matter how far from the truth such a statement would be, politically it is something the Republicans will make stick.
The chances of another terrorist attack may or may not be small, but, it is a risk that Obama and the Democrats cannot afford to take. Obama cannot afford to be on the side of the fence saying "I know hundreds/thousands of people are dead, but... Bush and the telecoms were breaking the law". Trust me. No one will want to hear a nuanced argument about the 4th Amendment at that time. And because of that, Obama and the Dems cannot afford to be caught on that side of the fence, no matter how principled of a stand it may be to not support the current bill.
The key issue that Obama has zeroed in on is whether or not the FISA court has oversight and review of the so-called wiretapping program. To the extent that the FISA court is codified/re-codified into law to encompass current methods of surviellance, then I for one am willing to give Obama and the other Dems the benefit of the doubt.