Hillary has strong support from Democrats, and Romney has strong support from Republicans. It seems like Obama and McCain have the same support base - moderates from both parties. If McCain had decided to drop out of the race before Tuesday, where would all of his votes have gone instead? I’d bet that Obama would be stomping Hillary with the moderate votes that would have gone his way had McCain not gotten so popular with moderate voters.
To a degree they’re all competing against each other. But there’s only so much realistic overlap.
Are there some voters who, in states with open primaries (not every state has open primaries, some don’t have primaries at all) that would have voted for Obama had McCain not been on the ballot? Sure, there’s at least some voters out there like that.
However since there are closed primaries in some states, and since people still tend to vote more often in their party’s primary than they do the other party’s, I think it is most likely that Obama and McCain’s chief competitors in the primary races continue to be the opposing candidates within their party.
You can still vote for anyone you feel like in states with closed primaries. It just takes an extra 3 minutes to re-register as the party of the candidate of your choice. I did it myself, actually.
In my State you have to re-register well in advance. I seriously doubt very many people do this just to vote for one particular person in a particular primary election. You’re an outlier, not a trend.
By and large McCain may be competing with Obama for some of the same types of voters, but not the same actual voters except for a very small percentage.