What would realistically be the most electable Democratic or Republican ticket?

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In a general election I think Bullock/Booker or Booker/Bullock would be formidable. Bridging the urban/rural divide imo is more important than bridging the progressive/liberal divide. You could replace Booker with Kamala Harris on either of those tickets as well to get the same outcome (while winning more progressives).

As of right now... This is the correct answer.  Bullock could maybe interchanged with Sherrod Brown.

Elizabeth Warren on a ticket is less electable than Hillary IMO.

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Assuming Trump is the Repub nom...
Some combo like:

Sherrod Brown/ Julian Castro
Al Franken/ Kamala Harris
Cory Booker/ (Heinrich or Brown)

... it think it will be smart to have some Geographical Diversity on the ticket & Someone from the SW (which is why I included Castro, Heinrich, Harris (although CA isn't exactly SW.. she would have SW appeal)

... although of the list above- Brown & Franken will be 69.... which make me hesitant (I think there is something to the stat that in modern era- every Dem elected President have been under 54 yrs old!) .. so Maybe switch the order (as older Dem candidates have been fine as VP picks)