The remaining two Democratic candidates both have a lot of support in the party. The base level of support for each of them (in primaries) seems to be about 40% or even more if they both campaigned in that state. Turnout is huge in any case. So where does momentum make a difference?

It doesn't, really. And if it does, it's workings are actually much more complex than political pundits usually assume. For example, there is the momentum-backlash we saw in New Hampshire and that we may see again in Ohio and Texas today. The fact that Obama is starting to look like the sure winner of this primary actually energizes the base of Clinton supporters more than it depresses them. Why is that? My guess is that a lot of her supporters simply have been waiting for this moment for too long - finally to cast a vote for a female President! They simply don't want to let that moment go too quickly and without a fight. This is essentially a visceral response (which, by the way, should in no way mean that one cannot support Sen. Clinton for purely intellectual reasons, but I believe that this is not how the majority votes in the end). So far, so good. This has made the primary interesting, after all.

But what about momentum for Clinton after possible wins today? How would that look like? Well, my honest opinion here is that it won't last very long.
Firstly, it seems that Sen. Obama has prepared for this situation. I point to the leak about 50 superdelegates - as hard to believe as it is, there could be some truth behind it - and the fact he kept his fundraising numbers secret. He always had to assume that he would lose today, after all, it is Sen. Clinton's official firewall. Even if some of his supporters (me included) became overconfident in the last week about his changes, you can expect his campaign to know better and prepare for the worst.
So secondly, the caucus in Wyoming and the primary in Mississippi strongly favor Obama - no Clinton momentum will stop him there, even if it may depress his margins. AA's will react to Clinton momentum as do women when it comes to Obama momentum - they will basically counteract it for the reasons explained above. After these two states, we would still see Obama ahead by roughly 100 delegates and a long period of waiting until Pennsylvania. Momentum won't last that long.

So, to conclude this short analysis - let's not talk about momentum. There is no momentum at this point of the primary. It's about delegates. And personally, I don't see a path to victory for Clinton unless Michigan and Florida get a re-vote or a huge scandal tears down Obama completely.