If both Romney and Bush run, Bush has a very difficult path to victory. Romney's quasi home state advantage in New Hampshire makes it very hard for another establishment candidate to get traction since they would probably have to win in the more conservative Iowa or South Carolina.
Though it's worth noting Romney is actually doing worse in NH now than he was at this point in the 2012 cycle, despite having lower name recognition then.
http://www.realclearpolitics.com/epolls/2012/president/nh/new_hampshire_republican_presidential_primary-1581.html