Notice that he has the greatest number of endorsements.  The area where he hasn't broken through is in

1.   Christian Fundamentalists, as they miscontrue his religion and values.  (He should address this.  Read    
     Why Romney Is Just Fine For Fundamentalists.
2.   Tea Party, as they misconstrue the benefits he will provide in lower taxes and the massively increased
     efficiency of the government that Mitt will bring about - the latter is necessary or lower taxes are just a
     fantasy.  He should be massively supported by Tea Party participants.

Chris Christie's reasons  (Video)  
Virginia Governor Bob McDonnell  - Consistent, results-oriented, conservative, most capable on jobs and cutting spending.
McCain's endorsement  
Senator John Thune (who was considered an excellent possible candidate for President)
John Sununu
President George H. W. Bush 
South Carolina Governor Haley  and Romney Not Part Of D.C. (video)
Elizabeth Dole And Bob Dole 
Former U.N. Ambassador John Bolton
Senator Ayotte
Jon Huntsman (See What Others Say page to see what his father says.)

State, By

Support of 40 Colorado business leaders 
And others in each state

Many more:  I couldn't find on Romney's site a central listing.  See in Press Releases.
                  See also, including some of the endorsements, What Others Say 

See also:

What Others Say