Has Fred endorsed McCain? If so, can you provide a link? I haven't seen anything on that. Not that it would surprise me at this point. With only McCain and Huckabee left in the race, who did you think he woud endorse? Huckabee is just as bad as McCain,just in other ways, and at least Fred is friends with McCain, so that would make sense.
http://blog.washingtonpost.com/the-trail/2008/02/08/fred_thompson_backs_mccain.html
Yes, but it was only after Romney dropped out and it became obvious that McCain was going to get the nomination. It's pretty obvious he only did it to unify the party now that the nomination contest is effectively over. Otherwise he wouldn't have waited this long.
I therefore don't see how anyone could hold this endorsement against Thompson, nor do I see how it detracts from his conservative credentials. Even though I'm not sure it's a good idea to support McCain in the general (I'm thinking it may be better for us long-term to lose this election), it's certainly a judgement call about which solid conservatives can honestly agree.