You lie.
Having run against Romney in the presidential primaries and with accompaning reports of bad blood between them, it was crucial for McCain to have a show of solidarity with Romney. Romney's endorsement released a lot of money that was on the sidelines and, more importantly, allowed party machinery to kick in with hundreds of the faithful going to work for McCain to one degree or another. It was Romney's appearance that clinched it for McCain, not Governor Palin's. If McCain was in trouble with the base, Romney would have been back.
Hayworth didn't help any when, at about the same time, he came out with his "I saw Jesus" remarks. In the West that is code for anti-Mormon.