Imagine you were running for President. You started out as the presumptive nominee for your party. You had all the establishment ties and the political machine was ready to mow down all comers on your behalf. But something unimaginable happened on the way to the convention floor: Some upstart built and managed a campaign far superior to yours and left you on the outside looking in. You fought and fought right down to the wire but still couldn't get the nomination.
Now imagination that you felt entitled to the Oval Office. Imagine as if it had been something you'd been promised for 10 years or more, say by a relative who had sat there. You might think that this young whippersnapper should be taken down at any cost, even if it meant destroying your own political party for the next 10-20 years. Assume too that you are impetulant and prone to fits of temper when you do not get what you want.
Your party has turned its back on you and your birthright. So what do you do?
Perhaps you start angling to the be the VP candidate for the other party. This could never happen of course. The other party hates you. They hate you more than they hate sin and vice (at least publicly). But what they fear most is this young upstart, because they are not really sure if they can beat him. The old party is not sure if they can take on this bulwark of the younger generations without looking old, stuffy and perhaps even racist.
And a large number of your supports would cross party lines to follow you, because you are essentially a Democrat in name only (DINO). So DINO that you are, you would be willing to cross lines to hold the VP seat as the first woman ever to do so, knowing that the presidential nominee is one fall or a few incoherent days from stepping down. Besides, it would keep that jerk who outcampaigned you from getting what is rightly yours.