It's the flip flops over the years. Its the 'Who is the real Mitt?'. Many people including I are just not sure. Is he for real or is he just playing politricks as usual? Dodging that tax return issue didn't work out well for him. That would hurt in a general debate.
Frankly, IMO the other side is wishing for Mitt to be the nominee. Forget all the pundits and talking heads saying he is O's biggest threat. He is not! They are only doing that to buoy him up while setting him up for a brutal attack in the general. O does not want to be matched up against a real down to the bone conservative who can articulate the conservative message and also excite the base. Remember Nov 2010?!! They know it can happen again in 2012. Conservatives are still a majority in America today according to Gallup. The rest can be convinced of truth if someone talks real a la Reagan. That's what the right candidate will do. Economy is tops but social issues still matter to many.
Right now though anybody except perhaps RP can do well against the machine. What do they have? Can't run on record so its class warfare and misinformation. Huntsman was always out. He never stood a chance. Even Harry Reid endorsed him
