Dole defended McCain to Limbaugh, and apparently Romney said this about the letter: 

It’s probably the last person I would have wanted write a letter for me. I think there’s a lot of folks who tend to think that maybe John McCain’s race is bit like Bob Dole’s race. That it’s the guy who’s next in line, the inevitable choice.

McCain then played the military service card and called Romney’s mocking of Dole “disgraceful.”  McCain is overreaching because he knows he has Romney on the ropes, but this is the sort of unforced error that Romney keeps making because he is always trying so very hard to prove that he’s not some moderate squish.  He just doesn’t know when to stop.  McCain’s campaign is a lot like Bob Dole’s, and will probably meet the same fate in the general election, but when you’re seeking a party’s nomination it is usually a good idea to respect the former nominees.