When did Presidential candidates start picking their running mates?

[darklordoftech]

As opposed to the convention delegates picking the running mate

[Mr. Smith]

1940, FDR's 3rd term.

Not sure why Stevenson decided to go back the conventions.

[President Johnson]

1940, FDR's 3rd term.

Not sure why Stevenson decided to go back the conventions.

Perhaps because he tought the candidate would enjoy greater backing in the party, though Wikipedia doesn't really give an answer to Stevenson's motive.

The open vice presidential question certainly helped Jack Kennedy to raise his national profile, as he took the lead in the second ballot and just narrowly fell short of a majority. He then lost the final ballot and gave a strong concession speech.

[Unconditional Surrender Truman]

As others have said, the last time a vice presidential nominee of a major party was chosen by a straight vote of the delegates was 1956, when Adlai Stevenson gave the DNC a free choice as a publicity stunt. Of course, presidential nominees had been involved before then to varying degrees. The full transition away from selection by the delegates and toward selection by the presidential candidate seems to coincide with the decline of national conventions in importance and the rise of primaries as the means by which a party's nominee is chosen.