It will accomplish a democrat win.
IF John jumps parties. He may very well run as an Independent.
No Democrat will vote for McCain the turncoat as headliner on a presidential ticket. His face value to Democrats is approximately [*] limited to his current role as a Democrat leaning kiss@ss from the Republican side of the aisle. His star value as a retread headliner approaches nada. [So sayeth SteveH-- fwiw ;-)]
As an indy (although I'm not certain the discussion started out this way), the media will support him only to the point he wicks away Republican support to Hillary!s advantage. To that end, he might shave off Ariz., but it doesn't have enough electoral votes to fill a half-full Tuscon spitoon (heaven help me, now I'm waxing Dan Ratherisms ;-).
Personally I think McCain's attraction as a ticket headliner is limited to sunbelt suburbs of Arizona and the MSM itself, in full political self-stimulatory mode.
(*) it just *might* make a difference if McC runs as second banana to Hillary. Maybe they can buy him off with a huge wad of offshore cash-- whatever it would take for him to swallow his ego and accept the attack dog role that comes with the VP slot. The MSM would give him all the face time he deserved in that role, multiplied by an order of magnitude by way of the traditional institutional bias. Hillary! needs him to "prove" (i.e., to the MSM, in a pretext kind of way) her "centrist" bona fides. All McCain would have to do would be to pick up the phone and give them his price... they'd probably meet it and consider themselves lucky he called them.
And that's IMHO how close we might be to a Hillary!/McCain ticket. The corollary, if the prediction were to come true, would be for McCain to bow out for health reasons soon after election, and replaced by someone else more Hillary!-compatible, such as Boxer or other political clone. Hillary!s strong suit is not patience (anyone recall don't ask, don't tell? not to mention dear departed Vince ;-) ;-)