Geography usually ranks near the top of the considerations that are commonly said to matter when selecting a vice presidential candidate. Political folk wisdom tells us that a would-be president seeks to balance the ticket by choosing a running mate from a different region of the nation, while a prospective VP from a critical battleground state holds the promise of attracting a favorite-son (or -daughter) vote that could tilt that state to his or her party, thus offering an advantage in the electoral college.
John F. Kennedy's choice of Lyndon B. Johnson as his running mate in 1960 is often treated as the model case of vice presidential selection. Johnson was credited in retrospect with helping to keep most of the South in the Democratic column—especially his home state of Texas, which Kennedy carried by less than 50,000 votes—at a time when the Democratic Party could no longer take the region for granted in presidential elections. When speculation turns in the late spring and summer of an election year to the identity of the vice presidential candidates, governors and senators from swing states like Ohio and Florida are often among the most frequently mentioned, reflecting the assumption that their presence on the ticket could potentially prove decisive to the outcome just as Johnson supposedly did.
As a rule, however, presidential candidates have not consistently elevated geography among other considerations when selecting running mates. Joe Biden (D 2008/2012), Sarah Palin (R 2008), Dick Cheney (R 2000/2004), and Joe Lieberman (D 2000) were all residents of small states that were already electorally safe for their party. On the other hand, the home states of John Edwards (D 2004) and Jack Kemp (R 1996) were almost certain to vote for the opposition regardless of their presence on the ticket. Paul Ryan (R 2012) did come from the battleground state of Wisconsin, but he had never represented more than one-eighth of the state's residents in Congress (after the election, Mitt Romney advisors revealed that Ryan was not chosen because the campaign thought that he would deliver Wisconsin to the Republican Party). The home state of Al Gore (D 1992/1996) was politically competitive but not widely considered pivotal to the national outcome.
Since Johnson in 1960, two other Texans have been nominated for vice president: George H. W. Bush (R 1980/1984) and Lloyd Bentsen (D 1988). Geography played a role in the selection of both, though other considerations seemed to matter more; both Bush and Bentsen were identified with rival party factions to the presidential candidates who chose them, and were placed on the ticket largely to promote party unity. Despite Ohio's longstanding importance in the electoral college, no Ohioan has appeared on a national ticket since John W. Bricker served as Thomas E. Dewey's running mate in 1944. Florida, a more recent swing state, has yet to be represented by a presidential or vice presidential nominee of either party.
It appears, then, that modern presidential candidates seldom choose running mates whom they believe will help them carry a specific state—much less an entire region. Indeed, there is little evidence that parties normally perform significantly better in the VP's home state compared to what they would otherwise expect (most presidential nominees, in contrast, attract a reliable favorite-son bonus of a few percentage points in the state's popular vote).
Of the major potential running mates for Hillary Clinton floated so far in the media (Tim Kaine, Elizabeth Warren, Julian Castro, Cory Booker, and Tom Perez), only one—Kaine—resides in a battleground state. Similarly, the apparent field of candidates for VP on the Republican ticket (Newt Gingrich, Chris Christie, Mike Pence, Jeff Sessions, and Joni Ernst) includes a single swing-state representative (Ernst, though Iowa casts only 6 electoral votes). This recent article suggests that somebody in the Trump campaign—probably pollster Kellyanne Conway—is talking up Pence by portraying his selection as allowing Trump to "shore up the Rust Belt," but there is little reason to believe that many voters outside Indiana (a state that is normally safe Republican territory) would care very much about Pence's presence on the ticket.
If Kaine or Ernst is nominated for the vice presidency this year, it is probable that political geography played a role in boosting them over other potential candidates. But recent history suggests that most presidential nominees view the potential electoral impact in a particular state or region as sufficiently modest that it does not outweigh other important factors relevant to the choice of running mate, from ideological positioning and political experience to campaign skill and personal rapport. We should not be surprised, then, if the 2016 presidential candidates follow this pattern by selecting running mates from safely red or blue states rather than electoral battlegrounds.