McCain is a Republican. He supports his party. The President is the leader of his party. Therefore, it is not all that odd that he will participate in a positive way in the campaign.
McCain is also a member of the Senate. He has said that he holds both Kerry and Edwards in favorable regard because they, too, are members of the Senate. He has said favorable things about both men.
I think you will find that McCain will not do any dirty work in this campaign.
Someone said that McCain is in the process of applying for the position of Secretary of State. When I first read that, I was thinking that there can be little doubt that Powell will not be part of a second Bush term (even if he denied the rumors last year). But also, McCain could be bucking for a position in a Kerry administration. It seems every adminstration has one or two members from the opposition (for example, Bush-Tenet or Clinton-Cohen).
What is sneaky, is how the Bush Adminestration is claiming that McCain was Kerry's first choice. Spin Spin Spin.
Mike