There are a couple differences between George W. Bush. John McCain, and Mitt Romney, but you end up in the same exact place.
George W. Bush said exactly who he was and what he stood for, and Republicans embraced him as a conservative. He had a set of well-defined principles, he ran on them, he won, and he governed according to them. Things had not gotten to the point yet when conservatism was "rediscovered" by anything approaching a majority of Republican voters. To most Republicans, Bush was a conservative, until events transpired that caused people to educate themselves as to the definition, and then compare him to it and found him severely lacking.
John McCain offered Bush without the principles or personality. He talked conservatism, but he had a record a thousand miles long of backstabbing conservatives at every opportunity. Republicans know him, and wanted none of it.
Mitt Romney offers the rhetoric of conservative principles with a liberal record a thousand miles long, and no evidence to demonstrate that he believes anything but the opposite of what he says - and if possible, he has less personality than John McCain, and certainly less authenticity as he mixes with the peasantry.
Which comes to the similarity in the end result. The main difference between Mitt Romney, John McCain, and George W. Bush is that Mitt Romney has not yet had the opportunity to stab conservatives in the back. And that is no difference at all.