The reason is simple, if McCain is President he and Republicans will be blamed for everything that goes wrong (and rightfully so) and the Democrats will be lauded for everything that goes right.
If Hildebeast is President, she and the Dems will go after each other and the remaining Republicans will actually set the agenda. Very little will happen and that is a good thing.
The moonbats will be all over her ass on day two if she doesn't announce the immediate withdrawl of our troops from Iraq in her inauguration speech. An opposition to her within her own party will set up immediately, all we have to do is get the Republicans in Congress to quickly shift to their side in opposing her legislation, like we did last fall on the Kucinich impeachment bill.
I'd find it more profitable to tell my congresscritters to oppose Hillary than to oppose McCainiac.