Romney won’t be a powerful voice - he already is disliked by many in the party nationally. When he tries doing the maverick thing the only support he will get is from from the left which will further erode any support he has.
He’ll have plenty of company.
Heller, Alexander, Graham, McCain, Murkowsky, Hoeven, Rubio, Tillis, Burr, Lankford, Gardner, Cornyn, Cochran, Collins, Sasse, Kennedy, Shelby, Ernst, Blunt, McConnell, Wicker, Portman, Isakson, Johnson, Toomey, Rounds, Thune, Gaines, Barasso and Enzi are all Bush League Republicans.