Clinton/Warren (D) vs. Trump/Webb (R)

[Frodo]

Let's suppose Jim Webb does a Zell Miller (he already has made clear he prefers Donald Trump over Hillary Clinton), and becomes Trump's running-mate. 

How would the race play out between the two tickets? 

[Frodo]

Won't happen.

Trump doesn't care about GOP unity, so why wouldn't he pick Jim Webb?  And as for Hillary, I wouldn't rule her out doing the unexpected (like Bill did with Al Gore in 1992) and doubling down with another woman on the ticket.  

[Frodo]

Everyone if Webb actually had a following amongst these supposed conservative working class Democrats he would've been able to poll more than the margin of error in any poll. He is irrelevant. Totally and completely. He's also probably the third most responsible person for the passage of Obamacare (after the late Arlen Specter and Al Franken) so yeah, not exactly the best choice to win over disaffected conservatives.

Way to contradict yourself in one paragraph. 

[Frodo]

OK simple question: If Webb is such a game changer why wasn't he able to poll even above the margin of error?

He didn't have the platform because he showed up at the wrong debate. He'd be the perfect foil for Trump - experience in elected office, service under Reagan and outstanding military record. They're both mainline Protestant, share populist ideology, have foreign-born wives and have cross-party appeal. At the very least, he'd make a fine surrogate.

Republicans would've liked this?

pro-choice Democrat with a liberal voting record who supported Obamacare and endorsed/campaigned for Obama twice?

If they are willing to forgive and overlook Donald Trump's many lapses from conservative orthodoxy just so they can stick it to 'The Establishment', why not Jim Webb's?  

[Frodo]

OK simple question: If Webb is such a game changer why wasn't he able to poll even above the margin of error?

He didn't have the platform because he showed up at the wrong debate. He'd be the perfect foil for Trump - experience in elected office, service under Reagan and outstanding military record. They're both mainline Protestant, share populist ideology, have foreign-born wives and have cross-party appeal. At the very least, he'd make a fine surrogate.

Republicans would've liked this?

pro-choice Democrat with a liberal voting record who supported Obamacare and endorsed/campaigned for Obama twice?

If they are willing to forgive and overlook Donald Trump's many lapses from conservative orthodoxy just so they can stick it to 'The Establishment', why not Jim Webb's?  

Some of them are. There's heavy resistance to Trump among the movement conservatives, evangelicals, the establishment, etc. for that very reason. Picking Webb as a VP would alienate them even further. And if Webb ran as a Republican to begin with, Trump would've already sucked up all that oxygen anyway.

Besides, let's not forget that Trump got his fame among Republicans for his birtherism. He might've been a liberal at certain points, but that pales in comparison to voting for Obamacare or outright endorsing the communist Muslim twice.

What you say makes sense, but Trump has already stated that he doesn't give a damn whether they support him or not.  So that leads me to think that he is definitely open to picking a running-mate as unorthodox as himself.