I too don’t understand why Bachman refused. I know she is not afraid of him. Yes, Trump is a powerful and smart businessman with a lot of common sense, very few of those types around anymore.
Bachmann seems to run interference for Romney on stage and off. This is no surprise that this decision aligns with his early pass on Trump.
One might as well attend a debate sponsored by Obama. Trump is threatening to run "third party", which will reelect Obama.