I can't. Once upon a time, I had no doubt that whoever won the Dem primaries would be the nominee. Now, I'm honestly not so sure. They are perfectly able to nominate someone else if they so choose - and definitely if Kerry himself is somehow persuaded to drop out (or dies) - and I can't help but think that the Democrats are aware of the option..
BTW, I gave some thought to what would happen if Kerry dropped out after the convention. So long as it was before the state ballot deadlines [in September], then the Democratic Party could proceed to nominate someone else by whatever procedure they agreed upon. I imagine that they would reconvene another convention.
If he dropped out after that (or died - I've actually wondered about this before) then they would have two options.
First, they could again nominate someone else and petition the legislatures and the courts state-by-state to permit access to the ballot (it would probably end up in the Supreme Court for at least some number of states).
Second, they could just run the campaign & election with his name on the ballot, which would be much more complicated. This would definitely be the case if a presidential nominee died, say, the night before the election.
Anyhow, if Bush received an electoral majority, then no problem things just carry on. If Kerry (or whatever is on the ballot line) receives a majority, then things get very interesting. Those electors become uncommitted and they can then cast their vote for whoever they want, even for Kerry or whatever. If a majority of them cast their votes for one candidate (say, Kerry's VP nominee) then that person becomes President (so long as he or she is eligible). If a majority of these now-uncommitted electors can't agree (in other words, if no one gets a majority) then it just goes to the U.S. House of Representatives (and the Senate elects the Vice President).
If none of that works out then the line of succession kicks in and the Speaker of the House becomes President.